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Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

Laboratory Performance 

Evaluation of Residential 

Integrated Heat Pump Water 

Heaters

B. Sparn, K. Hudon, and D. Christensen

National Renewable Energy Laboratory

Technical Report

NREL/TP-5500-52635

Revised June 2014
NREL is a national laboratory of the U.S. Department of Energy

Office of Energy Efficiency & Renewable Energy

Operated by the Alliance for Sustainable Energy, LLC

This report is available at no cost from the National Renewable Energy

Laboratory (NREL) at www.nrel.gov/publications.

Contract No. DE-AC36-08GO28308

National Renewable Energy Laboratory

15013 Denver West Parkway

Golden, CO 80401

303-275-3000 • www.nrel.gov

Laboratory Performance 

Evaluation of Residential 

Integrated Heat Pump Water 

Heaters 

B. Sparn, K. Hudon, and D. Christensen

National Renewable Energy Laboratory

Prepared under Task Nos. WTN9.1000 and ARRB.2204

Technical Report

NREL/TP-5500-52635

Revised June 2014
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Cover Photos: (left to right) photo by Pat Corkery, NREL 16416, photo from SunEdison, NREL 17423, photo by Pat Corkery, NREL 

16560, photo by Dennis Schroeder, NREL 17613, photo by Dean Armstrong, NREL 17436, photo by Pat Corkery, NREL 17721.
iii

Note 

The work presented in this report does not represent performance of any product relative to 

regulated minimum efficiency requirements. 

The laboratory and/or field site used for this work are not certified rating test facilities. The 

conditions and methods under which products were characterized for this work differ from 

standard rating conditions, as described. 

Because the methods and conditions differ, the reported results are not comparable to rated 

product performance and should only be used to estimate performance under the measured 

conditions. 

About the Revision

This report was revised in June 2014 to clarify that the rated performance of a water heater can 

only be determined at certified ratings laboratories.
iv

Executive Summary

The residential integrated heat pump water heater (HPWH) is an emerging technology that 

provides cost-effective energy savings compared to legacy products in domestic water heating. 

The heat pumps in these units are small refrigerant-based direct expansion systems, which absorb 

energy from the surrounding air and transfer it to water in an attached tank. This type of system 

is expected to have a high coefficient of performance (COP)—the ratio of the useful energy 

transferred to the electrical energy consumed— across wide ranges of operation. Electric 

resistance water heaters have a COP of nearly 1.0, but HPWHs are expected to provide annual 

COP values over 2.0. While HPWHs are not a new concept (the first HPWH was developed in 

the 1950s), several major water heater manufacturers have recently developed integrated HPWH

models, meaning that the heat pump and water tank come as a packaged unit with backup 

electric resistance elements. 

Researchers at the National Renewable Energy Laboratory (NREL) evaluated five integrated 

HPWHs in a laboratory setting to determine their performance in a range of ambient conditions 

and under different levels of use. The results describe how these products function, demonstrate 

that efficient operation is typical but limitations exist, and provide information to determine

reasonable expectations for the products. The performance testing was conducted at NREL’s

Advanced Thermal Conversion Laboratory, which is configured for high-accuracy performance 

measurement of thermal conversion systems. 

Four sets of tests were performed on each HPWH: operating mode tests, draw profile tests, 

performance-mapping tests, and reduced airflow tests. The operating mode tests were designed 

to tease out the control logic for each operating mode. The HPWHs had at least three operating 

modes: Heat pump-only mode, Hybrid mode, and Resistance-only mode. Hybrid mode uses the 

heat pump and backup resistance elements to balance the efficiency of the unit with the need for 

hot water, and the controls in this mode varied significantly between the different units. The 

draw profile tests subjected the HPWHs, while in Hybrid mode, to a simulated-use draw profile. 

The results from the draw profile tests were used to calculate system COP values for the HPWH 

running as a complete system. The performance-mapping tests were conducted under a number 

of different conditions ranging from a dry bulb temperature of 8°C to 40.5°C. Only the heat 

pump was allowed to heat the tank. These tests produced curves that determined the efficiency of 

the heat pump based on both the ambient air conditions and the average temperature of the water 

tank. The last set of tests evaluated how restricting the air flow to the HPWH, as may happen 

with a dirty filter, would affect performance. 

In general, all the HPWHs were very efficient in their typical operating range, with performance 

improving in warmer conditions. In colder conditions, performance goes down but the HPWHs 

are still more efficient than a standard electric water heater. The low temperature cutoff for heat 

pump operation is generally 45°F, and below that temperature the electric resistance elements are 

used exclusively. Comparisons of the heat pump COP from the performance-mapping tests and 

system COP from the draw profile tests are given below.
iii

Table ES-1. Comparison of System 

COP from Draw Profile Tests

HPWH Mfr.

COPsys

High Use

(Morning)

Low Use

(Evening)

Unit A 3.55 3.42

Unit B 1.21 1.81

Unit C 1.44 2.61

Unit D 3.85 5.37

Unit E 1.68 2.77

Some other factors that play into the performance of the HPWHs are the tank size and control 

logic. Two of the units tested had 80 gallon tanks, which allowed them to use their heat pump 

more, even in times of high demand. The control logic that determines when the backup 

resistance elements will be used can also have a big impact on performance. For instance, units 

that had a temperature sensor low in the tank could trigger the heat pump to turn on early in a 

draw, rather than waiting until the middle or top of tank filled with cold water. Tank size and 

control logic both play into the balance between efficiency and available hot water, as HPWHs 

have a slower rate of recovery if they are operating efficiently. Choosing the correct tank size 

and selecting an appropriate combination of set point and operating mode will ensure sufficient

hot water and significant reduction of water heating energy consumption. 

Figure ES-1. Comparison of heat pump COP 

curves in 14°C wet bulb ambient air
iv

Contents

Note ............................................................................................................................................................. iii

About the Revision .................................................................................................................................... iii

List of Figures ............................................................................................................................................ vi

List of Tables ............................................................................................................................................ viii

Definitions................................................................................................................................................... ix

Introduction ................................................................................................................................................. 1

Test Plan ...................................................................................................................................................... 2

Operating Mode Tests ................................................................................................................3

Draw Profile Tests .....................................................................................................................3

COPhpTests ................................................................................................................................4

Reduced Airflow Tests ..............................................................................................................5

Laboratory Setup ........................................................................................................................................ 6

Test Plenum ...............................................................................................................................6

Air-Side Equipment ...................................................................................................................6

Water-Side Equipment ...............................................................................................................8

Test Article Summary ............................................................................................................................... 10

Test Results............................................................................................................................................... 11

Uncertainty Analysis ................................................................................................................11

Unit A HPWH Summary .........................................................................................................12

Overall Impressions and Observations ..............................................................................12

Installation and Usability Considerations ..........................................................................12

Qualitative Test Results and Observations ........................................................................13

System Drawbacks .............................................................................................................18

Unit B HPWH Summary .........................................................................................................19

Overall Impressions and Observations ..............................................................................19

Installation and Usability Considerations ..........................................................................19

Qualitative Test Results and Observations ........................................................................20

System Drawbacks .............................................................................................................25

Unit C HPWH Summary .........................................................................................................27

Overall Impressions and Observations ..............................................................................27

Installation and Usability Considerations ..........................................................................27

Qualitative Test Results and Observations ........................................................................28

Anomalies ..........................................................................................................................33

System Drawbacks .............................................................................................................34

Unit D HPWH Summary .........................................................................................................35

Overall Impressions and Observations ..............................................................................35

Installation and Usability Considerations ..........................................................................35

Qualitative Test Results and Observations ........................................................................36

System Drawbacks .............................................................................................................40

Unit E HPWH Summary ..........................................................................................................41

Overall Impressions and Observations ..............................................................................41

Installation and Usability Considerations ..........................................................................41

Qualitative Test Results and Observations ........................................................................42

System Drawbacks .............................................................................................................47

Data Comparison and Analysis ............................................................................................................... 48

Conclusions............................................................................................................................................... 55

References................................................................................................................................................. 57
v 

Appendix A – Draw Profiles ..................................................................................................................... 58

Appendix B – Laboratory Schematics .................................................................................................... 61

Appendix C – Heating Capacity Curves.................................................................................................. 63
vi 

List of Figures

Figure ES-1. Comparison of heat pump COP curves in 14°C wet bulb ambient air............................ iii

Figure 1. Test plenum during normal testing (left) and during installation (right)............................... 6

Figure 2. Duct heaters used to control humidity (top left), heat exchanger to provide cooling 

(top right), evaporation pads (bottom left), and temperature and humidity measurement at 

inlet to plenum (bottom right) ............................................................................................................. 7

Figure 3. Laminar flow element used to measure airflow rates out of HPWHs.................................... 8

Figure 4. Inlet pressure transducer and thermocouple, outlet thermocouple, and dump 

solenoid valve on Unit B...................................................................................................................... 8

Figure 5. Thermocouple tree prior to installation.................................................................................... 9

Figure 6. Inlet turbine flow meters (left) and coriolis flow meter for condensate (right)..................... 9

Figure 7. Outlet turbine flow meters, proportioning valves, and solenoid valves used to 

control outlet flow rate and draws...................................................................................................... 9

Figure 8. Unit A 80 gallon HPWH installed in test plenum (left). A close-up of the 

compressor and refrigerant system in Unit A (upper right) and a side view of the 

evaporator coils and heat exchanger (lower right)......................................................................... 12

Figure 9. Draw Profile 1 for Unit A........................................................................................................... 15

Figure 10. COPhp traces for Unit A.......................................................................................................... 16

Figure 11. Effect of humidity on COPhp for Unit A................................................................................. 17

Figure 12. Effect of humidity on recovery time for Unit A .................................................................... 17

Figure 13. Heat pump COPhp traces for Unit A HPWH with reduced airflow ...................................... 18

Figure 14. Unit B in the test plenum (left) and the instrumented heat pump components (right).... 19

Figure 15. Draw Profile 1 for Unit B......................................................................................................... 22

Figure 16. COPhp traces for Unit B.......................................................................................................... 23

Figure 17. Effect of humidity on COPhp for Unit B................................................................................. 24

Figure 18. Effect of humidity on recovery time for Unit B .................................................................... 24

Figure 19. COPhp traces for Unit B with reduced airflow...................................................................... 25

Figure 20. Unit C in the test plenum (left), the compressor and coaxial heat exchanger 

(top right), and the water circulation pump, evaporator, and control wiring (bottom right) ...... 27

Figure 21. Draw Profile 1 for Unit C......................................................................................................... 30

Figure 22. COPhp traces for Unit C.......................................................................................................... 31

Figure 23. Effect of humidity on COPhp for Unit C................................................................................. 32

Figure 24. Effect of humidity on recovery time for Unit C .................................................................... 32

Figure 25. COPhp traces for Unit C with reduced airflow...................................................................... 33

Figure 26. Unit D, an 80 gallon HPWH, installed in the test plenum (left). A close up of the 

air intake (top right) and the heat pump (bottom right) are also shown....................................... 35

Figure 27. Draw Profile 1 for Unit D......................................................................................................... 37

Figure 28. COPhp traces for Unit D .......................................................................................................... 38

Figure 29. Effect of humidity on COPhp for Unit D................................................................................. 39

Figure 30. Effect of humidity on recovery time for Unit D .................................................................... 39

Figure 31. COPhp traces for the Unit D with reduced airflow ............................................................... 39

Figure 32. Unit E installed in test plenum (left). Some of the refrigerant system is shown 

(upper right), along with the blower that is designed to connect to a 6 in. duct (lower right) ... 41

Figure 33. Draw Profile 1 for Unit E......................................................................................................... 44

Figure 34. COPhp traces for Unit E .......................................................................................................... 45

Figure 35. Effect of humidity on COPhp for Unit E................................................................................. 46

Figure 36. Effect of humidity on recovery time for Unit E .................................................................... 46

Figure 37. COPhp traces Unit E with reduced airflow............................................................................ 46

Figure 38. COPhp comparison for five HPWHs at 14°C inlet wet bulb air temperature...................... 49

Figure 39. Heating capacity comparison for five HPWHs at 14°C inlet wet bulb air temperature.... 50

Figure 40. Graphical representation of Draw Profile 1.......................................................................... 58

Figure 41. Graphical representation of Draw Profile 2.......................................................................... 60

Figure 42. Water-side schematic ............................................................................................................. 61

Figure 43. Air-side schematic .................................................................................................................. 62
vii 

Figure 44. Heating capacity as a function of average tank temperature for Unit A ........................... 63

Figure 45. Heating capacity as a function of average tank temperature for Unit B ........................... 63

Figure 46. Heating capacity as a function of average tank temperature for Unit C ........................... 64

Figure 47. Heating capacity as a function of average tank temperature for Unit D ........................... 64

Figure 48. Heating capacity as a function of average tank temperature for Unit E ........................... 65

All figures in this report were created by NREL.
viii

List of Tables

Table ES-1. Comparison of System COP from Draw Profile Tests....................................................... iii

Table 1. Tests Performed on Each HPWH, With the Air and Water Conditions for Each Test 

Shown.................................................................................................................................................... 2

Table 2. Summary of the physical characteristics of the HPWH units tested .................................... 10

Table 3. Comparison of Operating Modes and Control Logic for All Five Units................................ 48

Table 4. Draw Profile System COPsys Values......................................................................................... 51

Table 5. Standby Heat Loss Coefficient for Each HPWH...................................................................... 52

Table 6. Recovery Rate Comparison vs. Electric Resistance Water Heater....................................... 52

Table 7. Cooling Capacity and SHR Comparison.................................................................................. 53

Table 8. Coefficients for Normalized COPhp Curve ............................................................................... 54

Table 9. Coefficients for Normalized Heating Capacity Curve ............................................................. 54

Table 10. Rated COPhp and Heating Capacity........................................................................................ 54

Table 11. Draw Profile 1 Details. Morning Segment Includes the Four Showers and the 

Evening Segment Contains Remaining Draws................................................................................ 58

Table 12. Draw Profile 2 Details............................................................................................................... 59

All tables in this report were created by NREL.
ix

Definitions 

Acronym Definition

AC Alternating Current

COPhp Coefficient of Performance of the Heat Pump

COPsys Coefficient of Performance of the System

DC Direct Current

DOE Department of Energy

DX Direct Expansion

EPA Environmental Protection Agency

HPWH Heat Pump Water Heater

ORNL Oak Ridge National Laboratory

SHR Sensible heating ratio

WFO Work for Others
1 

Introduction 

Advanced technologies for residential buildings are sought that provide broadly applicable, 

measurable, and cost-effective energy savings compared to legacy products. One significant 

opportunity for energy savings is domestic hot water heating, where an emerging technology has 

recently arrived in the U.S. market: the residential integrated heat pump water heater (HPWH). 

The heat pumps in these units are small refrigerant-based direct expansion (DX) systems, which 

absorb energy from the surrounding air and transfer it to water in an attached tank. This type of 

system is expected to have a high coefficient of performance (COP), which is the ratio of the 

useful energy transferred to the electrical energy consumed, across wide ranges of operation. 

Electric resistance water heaters have a COP of nearly 1.0, but HPWHs are expected to provide 

annual COP values over 2.0. 

The development of the HPWH began in the 1950s, when the Hotpoint Company, which later 

became a division of General Electric (GE), designed and built a HPWH intended for mass 

production (Calm 1984). The technology performed well, but was stricken with reliability issues. 

In the end, development efforts ceased because energy prices were low and there was little 

demand for the product. Fueled by rising energy prices in the 1970s, HPWH products reemerged, this time backed with improved heat pump technology. Oak Ridge National Laboratory 

(ORNL) tested both add-on and integrated units in 1982. Add-on HPWHs have a heat exchanger 

that is pushed into an existing hot water tank, often through the drain port. Integrated HPWHs 

are factory integrated with a water tank and backup resistance heating elements. ORNL’s field 

and laboratory testing showed that HPWHs use about half the energy to heat domestic hot water 

when compared to an electric resistance water heater (Levins 1982).

Despite this promising research, few HPWHs were sold once energy prices fell sharply in the 

1980s. It was not until the turn of the century that a resurgence in the technology began again. 

An Australia study in 2001 used a TRNSYS model based on test results from three HPWHs to 

show that the annual COP for an integrated HPWH was 2.3, which translated to annual energy 

savings of 56% (Morrison 2003). The technology was market ready, but a 2004 study identified 

cost, consumer awareness, and contractor technology perceptions as barriers to market 

acceptance in the United States (Ashdown 2004). Meanwhile, foreign markets embraced the 

technology. For example, models such as the EcoCute became prevalent in Japanese markets 

(Hashimoto 2006). 

Backed by the U.S. Department of Energy (DOE) and the Environmental Protection Agency 

(EPA), integrated HPWH technology was added to the ENERGY STAR® program in 2008 (CEC 

2011). This encouraged key manufacturers to revisit integrated HPWHs, resulting in those now 

available on the U.S. market. 

In this report, we present a laboratory evaluation of the five integrated HPWHs available in the 

U.S. market today. The results describe how these products function, demonstrate that efficient 

operation is typical but limitations exist, and provide information so reasonable expectations for 

the products can be determined. Performance testing occurred at the National Renewable Energy 

Laboratory (NREL) Advanced Thermal Conversion Laboratory, which is configured for highaccuracy performance measurement of thermal conversion systems and was used to explore 

HPWH performance across the full range of operating conditions. 
2 

Test Plan

The test plan for this project can be broken down into four sections. A summary of this test plan 

can be found in Table 1 and includes the air and water conditions, and operating modes associated 

with each test run during this experiment. The test plan was originally written by Ecotope, with a 

few additional tests added by NREL, and the complete list of tests performed is shown in Table 1. 

Following the summary are detailed descriptions of each of the tests that were performed. 

Five integrated HPWHs were tested: the A. O. Smith Voltex hybrid electric HPWH, the GE 

GeoSpring water heater, the Rheem Hybrid Electric water heater, the Stiebel Eltron Accelera 300 

HPWH, and the Air Generate AirTap Integrated HPWH. These will be referred to from this point 

on as Unit A, Unit B, Unit C, Unit D, and Unit E, respectively. The testing took place between 

October 2010 and May 2011, and occurred in three rounds. Two HWPHs were tested at a time 

during each round of testing. Unit B and Unit C were tested during the first round, Unit A and 

Unit D during the second round, and Unit E in the final round. 

Table 1. Tests Performed on Each HPWH, With the Air and Water Conditions for Each Test Shown

Test Name

Dry 

bulb 

(°C/°F) 

RH

Inlet 

Water 

(°C/°F)

Tank Set 

Point

(°C/°F) 

Airflow Operating Mode

1. OPERATING MODE TESTS

OM-67 20/67.5 50% 14/58 57/135 100% All Factory Modes

OM-95 35/95 40% 14/58 57/135 100% Hybrid Modes

OM-47 8/47 73% 14/58 57/135 100% Hybrid Modes

2. DRAW PROFILES

DP-1 20/67.5 50% 7/45 49/120 100% Factory Default

DP-2 20/67.5 50% 7/45 49/120 100% Factory Default

3. COPhp CURVE DEVELOPMENT – PERFORMANCE MAPPING

COPhp -47 8/47 73% 2/35 57/135 100% Compressor Only

COPhp -57 14/57 61% 2/35 57/135 100% Compressor Only

COPhp -67 20/67.5 50% 2/35 57/135 100% Compressor Only

COPhp -77 25/77 40% 2/35 57/135 100% Compressor Only

COPhp -85 29.5/85 42% 2/35 57/135 100% Compressor Only

COPhp -95 35/95 40% 2/35 57/135 100% Compressor Only

COPhp-95 dry 35/95 20% 2/35 57/135 100% Compressor Only

COPhp -105 40.5/105 42% 2/35 57/135 100% Compressor Only

COPhp -105 

dry 40.5/105 16% 2/35 57/135 100% Compressor Only 

4. REDUCED AIRFLOW

AF-1/3 20/67.5 50% 2/35 57/135 66% Compressor Only

AF-2/3 20/67.5 50% 2/35 57/135 33% Compressor Only
3 

During the second round of testing, OM-47 was not performed. During the third round of testing, 

the OM-95, OM-47, DP-3, COPhp -105 dry, and AF-1/3 tests were excluded to shorten the 

testing schedule by removing low-value tests. 

Operating Mode Tests 

The operating mode tests were designed to discover the control strategies for each water heater 

in each mode of operation. Each test began with the water heaters full of water at the “Outlet 

Water” set point and the inlet air temperature at 20°C dry bulb. A draw was initiated and 

continued until the compressor turned on. The draw was then stopped and the unit was allowed 

to recover. A second draw was performed for the same air conditions and set point. This second 

draw was allowed to continue until the electric heaters, if possible, for that operating mode came 

on, or until 80% of the tank volume had been drawn. The units were then allowed to recover. 

In addition to the 20°C dry bulb air condition, this procedure was followed in Hybrid mode only 

for air at 8°C dry bulb and 35°C dry bulb. This included the Hybrid mode for Unit A, the Hybrid 

and High Demand modes for Unit B, the Energy Saver and Normal modes for Unit C, on for 

Unit D, and Auto Mode for Unit E (Note: Units A, D, and E were not tested at the 8°C dry bulb 

air temperature because the results from this air condition provided little useful information 

during the first round of testing). Each mode of operation was tested at both the hottest water set 

point (60°C for Units A, B and D and ~57°C for Units C and E) and the standard 49°C set point. 

Draw Profile Tests 

Two draw profiles were used to challenge the water heaters with high volume draws and low 

volume draws over the course of several hours. The draw profiles, suggested by Ecotope, Inc., 

are based on NREL work (Hendron 2010), and a tabular representation of each profile can be 

found in Appendix A. Each test began with a full tank at the standard 49°C (120°F) set point. 

Draw Profile 1 contained a “morning” segment and an “evening” segment. The morning segment 

contains four showers over the course of an hour to test the performance in high demand 

situations. The evening segment of Draw Profile 1 contained a range of flow rates and draw 

durations to simulate the variety of hot water draws that can occur, such as dishwashing and food 

preparation. After the morning draws were completed, the units were allowed to recover fully 

before starting the evening segment. Full recovery was deemed complete when all the heating 

elements, either the electric resistance elements and/or the heat pump, were turned off by the 

water heater’s controller. Draw Profile 2 consisted of many short draws and was allowed to run 

uninterrupted. Depending on tank size and recovery rate, these draw profiles have the potential to 

deplete the hot water reserve, resulting in warm or even cold water being delivered to the users. 

For the results of these tests, “hot” water was defined as anything above 40.5°C (105°F). 

The results from the Draw Profile tests were used to calculate system COP values for the HPWH 

running as a complete system. This means that if the HPWHs used back-up electric resistance 

heat, the COP value would reflect this. For the remainder of this report, the system COP will be 

referred to as COPsys. The method used to calculate COPsys values is defined in the following 

subsection. 
4 

The Draw Profile tests also provided an opportunity to measure the standby heat loss coefficient 

(UA). UA can be calculated anytime that the water heater is idle, neither heating the tank nor 

experiencing draws. The standby heat loss coefficient was calculated as: 

UA = �Tmax − Tf,stby�VstρCP

τstby�Tt,stby − Ta,stby�

where Tmax is the maximum mean tank temperature after the tank has fully recovered, Tf,stby is the 

mean tank temperature at the end of the standby period, Vst is the measured volume of the 

storage tank, ρ is the density of stored hot water at the average of Tmax and Tf,stby, CP is the 

specific heat of the stored water at the average of Tmax and Tf,stby, τstby is the elapsed time of the 

standby period, Tt,stby is the average tank temperature over the entire standby period and Ta,stby is 

the average ambient air temperature over the entire standby period. 

COPhpTests

The energy efficiency gained from a HPWH is due to its ability to transfer more heat from the air 

into water than the energy consumed by that transfer process. Thus, a series of tests were 

conducted to examine refrigerant system performance alone. To ensure the resistance heaters did 

not turn on, they were electrically disconnected for the units without a heat pump-only mode. 

The results from these tests were used to calculate a COP for the heat pump operating alone. The

COP for the heat pump operating alone will be referred to as COPhp for the remainder of this 

report.

Unlike the other tests, the COPhp tests began with a tank full of cold water. Each tank was empty 

to start the day and the tanks were filled with 3°C (or colder) water. Once the water heaters were 

full of cold water, the units were turned on in their most efficient operating mode: Efficiency for 

Unit A, eHeat for Unit B, Energy Saver for Unit C, and Econ for Unit E. Unit D has only one 

operating mode, so this unit was turned on without selecting a mode. The test was deemed 

complete when the tank set point was achieved. 

To run these tests using the heat pump only, Units A and C needed to be modified because the 

compressor is disabled when the water temperature is below 27°C in the case of Unit C and 14°C 

in the case of Unit A. Also, the electric resistance elements were disabled for Units C and D 

since both these units can use their heating elements in conjunction with the heat pump, even in 

their most efficient operating mode. The modifications needed to ensure compressor-only 

operation are discussed in more detail in the summary sections for each HPWH. 

As mentioned previously, the results of these tests were used to calculate the COPhp for the 

operation of the heat pump alone across a wide range of air and water temperatures. The 

coefficient of performance is the measure of useful energy transferred to the water (output) 

divided by the input energy to the system (supplied work). The equation used to calculate COP is 

given below. 

COP = 

Qthermal

WInput

= m ∙ Cp ∙ (∆T)

WInput
5 

In this equation, m is the mass of water in the tank, Cp is the specific heat of water, and ΔT

represents the difference in average tank temperature over a given time step, which, in this case, 

is 1 minute. Average tank temperature is calculated as the arithmetic mean of the six 

thermocouples mounted inside the tanks. The supplied work consists of the overall input energy 

to the unit, which includes the energy used by the heat pump, fan, electronic display, and 

circulation pump, if applicable. The heating capacity of the compressor, a useful quantity for 

modeling purposes, was also captured during this test. Heating capacity is defined as: 

Heating Capacity = 

Qthermal

t

where Qthermal is the thermal energy from the previous equation and t is the time step. The 

heating capacity for all units can be found in Appendix C. 

The COP of the entire system, COPsys, values were calculated for the draw profiles mentioned 

above. This calculation is different from the COPhp calculations in that m is the mass of the 

volume drawn and is determined using the volumetric flow rate and ΔT is the difference between 

the outlet and inlet water temperatures during draws. The supplied work term also includes the 

energy used by the electric resistance elements. 

NOTE 1: Standby losses are not inherently included in the thermal energy term of the COPsys

calculations because the inlet and outlet temperatures are used to calculate the thermal energy. 

This is in contrast to COPhp, which uses the average temperature within the tank to calculate 

thermal energy, and therefore takes into account standby losses. In order to include the standby 

losses in the thermal energy term of the COPsys calculations, the UA values calculated during the 

Draw Profile tests were used to determine the thermal loss associated with the tank. This loss 

was then added to the thermal energy term in the COPsys equation. 

NOTE 2: The performance of the heat pump depends on both temperature and humidity, so 

graphs showing the COPhp curves reference the inlet air temperature as a wet bulb temperature

(see Figure 10, Figure 16, Figure 22, Figure 28, and Figure 34). 

NOTE 3: Any data for COPhp presented in this report inherently includes standby losses. 

However, when COPhp is used in future models, the curves will be modified to remove the 

standby losses using the UA values calculated during the Draw Profile tests. 

Reduced Airflow Tests 

The setup for these tests was identical to the COPhp tests described above, but the filter area was 

restricted for each water heater. Tape was used to block one-third and then two-thirds of the 

filter’s surface area for these tests. These tests were designed to see the impact on performance if 

the filter was never cleaned or if something was obstructing the air intake area. COPhp curves 

were calculated for these tests using the same method as described above for the COPhp tests.
6 

NREL/PIX 18678 and 18670, Credit: Kate Hudon 

Laboratory Setup

This section describes the laboratory setup used to conduct the above test plan. The setup is 

divided into two sections: the air-side of the experiment and the water-side of the experiment. 

Real-time measurements taken on the two sides of the experiment were used to accurately 

control the air and water conditions at the inlet to the experiment and were used to determine the 

performance of each test article. Schematics for both setups can be found in Appendix B. 

Test Plenum

Each test article was enclosed in an insulated air plenum. During the first round of testing, Units 

B and C were enclosed in the same test plenum that was physically partitioned (but not air 

sealed) to ensure that the operational cycle of one HPWH did not affect the operation of the other 

one. Units A and D were tested together during the second round of testing. The third round of 

testing was conducted on Unit D alone. 

Figure 1. Test plenum during normal testing (left) and during installation (right)

An inlet duct was attached to the test plenum, creating a means to control the environment from 

which the heat pump drew air. At all times, the plenum’s inlet airflow was greater than the total 

airflow used by the HPWHs, to allow excess conditioned air to exit the plenum via a bypass air 

duct. As a result, uniform ambient conditions were assured in the proximity of the tanks. When a 

heat pump was operating, its exhaust air was collected in an outlet plenum connected to an outlet 

duct. The outlet plenums were not connected to each other. 

Air-Side Equipment

A schematic of the air conditioning equipment that was used during this project can be found in 

Appendix B, Figure 43. The conditions specified in the test plan require that accurately

controlled conditioned air be supplied to and around the test article. Across the range of tests, the 

air within the test plenum needed to be heated, cooled, and/or humidified to achieve the desired 

inlet air conditions. 

Inlet duct

Inlet duct

Partition 

Wall
7 

Moisture was added to the air via evaporation pads. Moisture addition was controlled by heaters 

located upstream of the evaporation pads. Once the required water content was attained, the air 

was either heated further by additional electric resistance heaters or cooled using a chiller and 

heat exchanger system. The temperature and humidity was measured in a duct directly upstream 

of the plenum inlet to ensure accurate inlet air conditions to the test articles.

Figure 2. Duct heaters used to control humidity (top left), heat exchanger to provide cooling (top 

right), evaporation pads (bottom left), and temperature and humidity measurement at inlet to 

plenum (bottom right)

Inlet airflow rate was measured using ASME standard flow nozzles located downstream of the 

evaporative pads. The inlet pressure was measured using four static pressure taps located in the 

test plenum. These pressure taps were physically averaged together prior to measurement. The 

temperature and humidity of the air exiting each of the test articles was measured in an outlet 

duct located directly downstream of the units. The outlet pressure was measured with static 

pressure taps located in each test article’s outlet plenum. Each outlet duct was routed to a laminar 

flow element for accurate measurement of heat pump airflow rate. Boost fans were used to 

overcome the pressure drop of laboratory equipment in the exhaust airstreams, thus preventing 

the test articles from experiencing any performance-degrading backpressure. 

NREL/PIX 18683, 18687, 18688, and 19464Credit: Kate Hudon 
8 

Figure 3. Laminar flow element used to measure airflow rates out of HPWHs

Water-Side Equipment

A schematic for the water-side equipment can be found in Appendix B. For each test, a steady 

and well-controlled inlet water temperature was required to emulate a range of water main 

temperatures. A large holding tank was preconditioned prior to each test and maintained at the 

desired temperature using a heater or chiller with a heat exchanger. An icemaker was used to 

rapidly reach colder inlet water temperatures. 

A water dump solenoid valve was actuated prior to the beginning of each draw to flush the inlet 

pipe with water at the desired “Inlet Water” set point temperature. The inlet water temperature 

and pressure was measured immediately upstream of the test article and the outlet water 

temperature and pressure was measured immediately downstream of the test article. Location of 

inlet and outlet thermocouples was based upon the DOE test specifications found in 10 CFR Part 

430, Subpart B, Appendix E (DOE 1998). The inlet and outlet water pipes were insulated to limit 

heat transfer between the pipes and their surroundings. 

Figure 4. Inlet pressure transducer and thermocouple, outlet thermocouple, and dump solenoid 

valve on Unit B

A thermocouple tree consisting of six thermocouples was placed within each test article to 

measure stratification in the tank. Care was taken to position these thermocouples at the center of 

uniform volumes of water, which was often a challenge due to the non-uniform interior profile of NREL/PIX 18679, Credit: Kate Hudon NREL/PIX 18666, Credit: Kate Hudon 

Dump 

Valve 

Pressure 

Transducer

Inlet and Outlet 

Thermocouples
9 

some tanks. The thermocouple tree construction was also specified by the DOE test specification

document (DOE 1998). These measurements were also necessary to calculate the performance of 

the test articles and to help understand their control logic. 

Figure 5. Thermocouple tree prior to installation

The inlet and outlet water flow rates were measured using turbine flow meters. The only method 

to accurately measure condensate flow and condensate density from the heat pump is a coriolis 

flow meter. A coriolis meter was attached to the evaporator drain pan of each test article to 

measure condensate production. The temperature of the condensate flow was also measured. 

Figure 6. Inlet turbine flow meters (left) and coriolis flow meter for condensate (right)

For the tests having a prescribed water draw profile, an electronically controlled proportional

valve was used. Draw profiles were preprogrammed and the turbine flow meter measurements at 

the test articles’ outlets were monitored during draws to ensure the correct flow rates. Prior to 

testing, a correlation was established between the percentage opening in the value and the 

resulting flow rate for each HWPH. This correlation was used to write draw profile programs 

that were tailored for each HPWH. The water from the outlet line was directed towards a 

laboratory drain. 

Figure 7. Outlet turbine flow meters, proportioning valves, and solenoid valves used to control 

outlet flow rate and draws

NREL/PIX 18684 and 18686, Credit: Kate Hudon 

NREL/PIX 18685, Credit: Kate Hudon 
10 

Test Article Summary

Five integrated HPWHs were tested: the A. O. Smith Voltex hybrid electric HPWH, the GE 

GeoSpring water heater, the Rheem Hybrid Electric water heater, the Stiebel Eltron Accelera 300 

HPWH, and the Air Generate AirTap Integrated HPWH. Again, these will be referred to as Unit 

A, Unit B, Unit C, Unit D, and Unit E throughout the report. All five units are considered hybrid 

HPWHs because they all have back-up electric resistance elements that can heat the water like a 

traditional electric water heater. While all units are similar in design and operation, each system 

is unique. The characteristics of all units are summarized in Table 2.

Table 2. Summary of the physical characteristics of the HPWH units tested

Nominal 

Capacity 

(Gallons)

Compressor 

Power (W)

Electric 

Element 

Sizes (kW)

Refrigerant Condenser 

Type 

Circulation 

Pump

Water 

Lines

Unit A 80 960 4.5 Upper 

2.0 Lower R-134a 

WrapAround 

Tank

No Side/

Horizontal 

Unit B 50 700 4.5 Upper 

4.5 Lower R-134a 

WrapAround 

Tank

No Top/ 

Vertical

Unit C 50 1000 2.0 Upper 

2.0 Lower R-410a 

Coaxial 

Heat 

Exchanger

Yes Side/

Horizontal 

Unit D 80 500 1.7 Upper R-134a 

WrapAround 

Tank

No Side/

Horizontal 

Unit E 66 800 4.0 Upper R-410a Immersed 

Coils No Side/Horizontal

Units B and C have a rated capacity of 50 gallons, while the Units A and D have a rated capacity 

of 80 gallons and Unit E has a tank with a rated capacity of 66 gallons. Measured tank capacity 

was about 45.5 gallons for Units B and C, 75 gallons for Unit A, 80 gallons for Unit D and 63 

gallons for Unit E. The compressors have slightly different power draws, with Unit D having the 

smallest compressor at 500 W and Unit C having the largest compressor at 1000 W. The 

combination of electric resistance elements also varies for each unit, with Unit B having two 4.5 

kW elements, Unit C having two 2.0 kW elements, and Unit A having a 4.5 kW upper element 

and a 2.0 kW lower element. Units D and E have only a single back-up resistance heater, but 

Unit E has a 4.0 kW heater whereas Unit D has a small 1.7 kW heater. Units A, B and D use R134a refrigerant and a wrap-around style of condenser. In contrast, Unit C uses R-410a 

refrigerant and pumps water from the bottom of its tank and through a coaxial heat exchanger 

that serves as its condenser. Unit E also uses R-410a refrigerant but its condenser is an immersed 

coil that sits inside the tank near the bottom. The inlet and outlet water lines are vertical and 

come through the top of the tank for Unit B, whereas all other units have horizontal inlet and 

outlet water lines that are found on the side of each tank. 
11 

The performance characteristics and control logic of each of these units also differ and will be 

discussed in more detail in the following section. 

Test Results

The results for each HPWH are described in the following sections. The specific test results are 

summarized and overall impressions are given for each test article. A list of suggested 

improvements is also provided where opportunity for improving performance was uncovered. 

The suggestions given are based on our understanding of expectations that will be placed on a 

HPWH in an American home. By incorporating all the best features across the five brands of 

HPWHs, each one individually could be improved. 

Uncertainty Analysis

An uncertainty analysis was performed for all major results presented in this paper. Standard 

error propagation was performed and the associated uncertainty for each measurement is 

presented alongside the major result (Taylor 1997). The error analysis presented takes into 

account sensor accuracy as provided by the manufacturers. The random error component could 

not be characterized since each test was only completed once. 
12 

Unit A HPWH Summary

Overall Impressions and Observations

Unit A has a large storage tank and control logic that turns on the heat pump after small draws

and is able to deliver hot outlet water for all tests performed. It has an efficient heat pump that 

can quickly heat the tank of water, even with an 80-gallon tank. The logic controlling the electric 

resistance elements in Hybrid mode is intended to quickly provide hot water when demand is 

high, but the majority of the heating load is provided by the heat pump. The engineers also chose 

a smaller lower resistance element that only operates in Electric Only mode to encourage the use 

of the Hybrid mode. The smaller lower element means that the Electric Only mode is equal to or 

slower at heating the full tank than Hybrid mode. Overall, this unit succeeds in providing hot 

water for all scenarios tested and can maintain a high level of performance, even at low ambient 

temperatures. 

Installation and Usability Considerations

Unit A’s inlet and outlet water lines are located on the side of the tank. The 80 gallon unit that 

was tested is nearly 7 ft tall. This could limit the possible installation locations in a home, 

especially in a retrofit situation. The 60 gallon version of Unit A is 5 ft 7 in. tall and may be a 

better choice if space is limited. Also, the 80 gallon unit tested requires an airflow rate of about 

500 cfm. This means that the installation requirement for a location with unrestricted airflow is 

Figure 8. Unit A 80 gallon HPWH installed in test plenum (left). A close-up of the compressor 

and refrigerant system in Unit A (upper right) and a side view of the evaporator coils and heat 

exchanger (lower right).

NREL/PIX 18675, 18673, and 18674, Credit: Kate Hudon 
13 

very important for Unit A. The air enters the unit from the left (when viewed from the front), is 

pulled across the evaporator coils located in the middle of the heat pump, and exits from the right 

of the unit. The air filter slides into the front of the top cover and is secured with a screw. 

Currently, Unit A is not available through a major retailer, but can be purchased through 

plumbing suppliers. 

The user interface displays information about the current set point, the heating elements currently 

being used, the current mode and ways to change the mode and set point, but the touch screen is 

not as responsive as it should be. Buttons may need to be pressed a few times before it responds. 

The operating mode controls are accessible and clearly labeled, and changing the set point is also 

obvious. There are three operating modes available: Efficiency, Hybrid, and Electric. There is 

also a Vacation mode that reduces the temperature set point to approximately 15°C. Changes 

made to operating mode take place almost immediately. When the heat pump is turned on, from 

off or as a result of a change in operating mode, the fan will turn on immediately and the 

compressor will turn on approximately 1 minute after the fan. 

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit A HPWH and the observations 

made during testing. 

Operating Mode Tests

Unit A has two thermistors affixed to the exterior of the tank (under the insulation layer) that are 

used to control the heat pump and electric elements. The thermistors are located at the same 

height as the backup resistance elements. The following equation, which was provided by the 

manufacturer, shows the average tank temperature estimate that is used to control this unit:

Ttank,mfr = �3 ∗ Tupper + Tlower�⁄4

In this equation, Tupper is the temperature measured by the upper thermistor and Tlower is the 

temperature measured by the lower thermistor. The upper thermistor is located near the upper 

heating element and the lower thermistor is located near the lower heating element. 

This quantity is monitored to dictate when heating is required. None of the heat sources—the 

heat pump, the upper electric element, or the lower element—can operate concurrently with 

another. Also, if the tank temperature is below 14°C upon initial startup, the heat pump will not 

run and the upper heating element will turn on instead. 

Operation Modes and description:

1. Efficiency Mode: The heat pump turns on when Ttank,mfr drops 5°C below the set point 

temperature and will run until Ttank,mfr is equal to the set point. This mode of operation 

uses the heat pump exclusively unless the air temperature is outside the operating bounds, 

7°C to 43°C, as defined by the manufacturer, or Ttank,mfr falls below 14°C. If either of 

these conditions occur, the upper heating element turns on until Tupper reaches set point. 

The heat pump will then be used to finish the heating cycle, which is concluded when 

Ttank,mfr equals the set point temperature. 
14 

2. Hybrid Mode: The heat pump will turn on when a 5°C drop in Ttank.mfr is detected and the 

upper heating element will turn on in place of the heat pump when Ttank.mfr has dropped 

by 10°C. The upper element will turn off when Tupper is at set point and revert to the heat 

pump to finish the heating cycle. 

3. Electric Only Mode: Will only use electric resistance heating elements to heat the tank. A 

small drop in Ttank,mfr (2°C) will cause the upper heating element to turn on. The upper 

element will remain on until Tupper is at set point. The lower element will then turn on to 

heat the rest of the tank. Because the lower heating element has a small heating capacity 

(2kW), this mode has no benefit over the Hybrid mode in either recovery time or 

efficiency. This mode should only be used if the heat pump is not performing correctly. 

NOTE: For most air conditions, the compressor provides more heat input to the tank of water 

than the lower resistance element. In the worst case scenario, they provide about equal heat 

input. According to the engineers at Unit A’s manufacturer, they chose the size of the lower 

element for this exact reason. They did not want the electric resistance mode to have a 

performance advantage over the Hybrid mode. This should ensure that Hybrid mode (or 

Efficiency mode) is used exclusively by homeowners. 

Draw Profile Tests

Draw Profile 1 was tested in two parts. During the “morning” segment of Draw Profile 1, four 

showers are simulated over the course of a little more than an hour. Unit A was able to maintain 

an outlet temperature of 49°C during all four showers, with the only decrease in outlet 

temperature occurring at the very end of the fourth shower. This is well above the criteria for 

“hot” water (40.5°C). The compressor turned on after the first draw and remained on for the rest 

of the morning segment until Ttank,mfr was equal to the set point temperature. The evening draws 

were also completed with no measurable dip in outlet temperature. The compressor came on 

about halfway through the evening portion and remained on until set point was achieved after the 

profile finished. Neither segment of Draw Profile 1 triggered the operation of the upper 

resistance element. 
15 

Figure 9. Draw Profile 1 for Unit A

Draw Profile 2 consisted of many small draws over a 6-hour period. Unit A performed well for 

this low-demand situation. The compressor turned on to reheat the tank about 4 hours into the 6-

hour profile. 

COPhp Tests

The results of the COPhp tests are plotted below in Figure 10. As expected, COPhp increases as 

the inlet air temperatures increases and the COPhp decreases as the water temperature in the tank 

increases. All tests were run in Efficiency mode to ensure that only the compressor was used to 

the heat the tank. Even at the lowest air temperatures, Unit A did not use its resistance elements. 

While COPhp was lower at the colder air temperatures, there was not a dramatic decrease in 

COPhp at the lowest air temperatures that would suggest there was icing on the evaporator coils. 
16 

Figure 10. COPhp traces for Unit A

The two warmest COPhp tests were repeated with the same inlet dry bulb temperature for high 

and low relative humidity. Figure 11 shows this comparison for the inlet dry bulb condition at 

35°C with the relative humidity at 40% and 19%. The corresponding wet bulb temperatures for 

these conditions are 24°C and 17°C, respectively. 

The results show that the performance improvements were not significant for the COPhp test run 

at the higher inlet wet bulb temperature. For the lower tank temperatures, a small increase in 

COPhp can be seen. However, as the average tank temperatures increases, the performance 

difference becomes insignificant. The tank recovery time is shown in Figure 12. This shows that 

the lower humidity reduced the recovery rate from 10.5°C/hr to 10.4°C/hr. These results show 

that humidity has a small, but not significant, impact on the performance of this unit. 
17 

Figure 11. Effect of humidity on COPhp for Unit A Figure 12. Effect of humidity on recovery time 

for Unit A

NOTE: To prevent situations where icing is more likely, the compressor will not run if the water 

in the tank is below 14°C. In order to run the COPhp tests beginning with 3°C water 

temperatures, the thermistors fastened to the outside of the tank were moved so that they would 

sense the air temperature, rather than the water temperature. At the lowest air temperature tested, 

8°C, the thermistors were wrapped in insulation and heated slightly to keep the compressor 

running. When tank set point was reached, the HWPH had to be manually shut off since these 

thermistors were also used to measure tank temperature and turn off power when set point is 

achieved. The thermocouple tree installed inside the tank for testing purposes was used to 

monitor when set point was achieved. 

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 13. The results of the onethird airflow blockage test show an insignificant decrease in performance (<1% reduction 

in COPhp). The results of the two-thirds airflow blockage test show a slight decrease in 

performance, ranging from ~6% to ~2 % over the course of the heating cycle. 
18 

Figure 13. Heat pump COPhp traces for Unit A HPWH with reduced airflow

System Drawbacks

Below is a list of system drawbacks that could be improved to enhance the capabilities of Unit 

A: 

1. The 80 gallon unit is very tall. The large tank capacity has many benefits from a 

performance perspective, but its physical size may limit the homes in which it can be 

installed. The 60 gallon unit is smaller and so should fit in more homes, but there will 

likely be differences in hot water availability under high demand. 

2. Higher airflow required. Unit A requires about 500 cfm of continuous airflow when the 

heat pump is operating. The installed location of this unit will need to accommodate this 

large airflow. Also, 500 cfm of cold air will likely be noticeable to the homeowners and 

could require ducting in some climates to prevent discomfort in the house during the 

winter months. 

3. Control panel touch screen is not sensitive enough. The touch screen on the control 

panel is not very responsive. We found that we often had to touch a button multiple times 

before the command was received. However, the layout of the control panel is simple and 

the icons used are intuitive. 
19 

NREL/PIX 18671, 18664, and 18665, Credit: Kate Hudon 

Unit B HPWH Summary

Overall Impressions and Observations

Unit B will use its heat pump to heat water the majority of the time, but two large electric 

resistance backup elements will turn on when faster recovery is needed. The compressor has a 

smaller capacity than most of the other units tested but it consistently operates across a wide 

range of conditions. The electric resistance elements were the largest of the five units in terms of 

combined power output, which allows Unit B to heat a full tank of water quickly when demand 

is high. The eHeat mode uses only the heat pump for maximum efficiency while the Hybrid and 

High Demand modes use a combination of heat pump and resistance elements to meet demand, 

which can provide the user with faster recovery times in high demand situations. Unit B can be 

operated with minimal energy demand for energy-conscious users but also provides a higher 

energy-use mode to meet high water heating demands. 

Installation and Usability Considerations

Installation of Unit B in our lab space was straightforward, without issue. The inlet and outlet 

water lines come directly out of the top of the unit, like most traditional gas or electric water 

heaters. The air filter lifts up from the top of the unit, and can easily be accessed for cleaning. 

The top cover can be removed without disturbing the water lines or power cable, providing 

Figure 14. Unit B in the test plenum (left) and the instrumented heat pump 

components (right)
20 

access to the heat pump components for maintenance or repair. The unit is presently available for 

purchase through Lowe’s.

The control interface on this unit consists of a LCD screen with a key pad and buttons used to 

change settings. The user must navigate through multiple menus to change the set point 

temperature or select an operating mode. Unit B can be operated in four modes: eHeat, Hybrid, 

High-Demand, and Electric Only. If modes are switched while the compressor or heating 

elements are running, the change takes place immediately. Initial start-up in eHeat or Hybrid 

mode results in the fan turning on immediately followed by the compressor, 2–3 minutes later.

The air enters the heat pump from the neck and sides and is pulled across the evaporator coils to 

the exit at the back of the unit using two variable-speed, direct-current fans. To measure the 

speed of the variable speed fans, an optical tachometer was installed on one of the fans. We also 

measured the speed of the second fan to ensure that they operate identically, which confirmed the 

information provided by the engineers from the manufacturer. The variable speed fans ramp up 

to compensate in times of reduced air flow and their speed also varied in response to changes in 

inlet air conditions. 

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit B and the observations made 

during testing. 

Operating Mode Tests

Unit B does not allow either of the two electric resistance elements to operate simultaneously 

with the other, nor with the heat pump. Below is a description of the control logic that was 

determined during the operating mode tests. According to the manufacturer, a thermistor located 

near the upper heating element is used as input to the unit’s controller. 

Operation Modes and Description:

1. eHeat Mode: The heat pump will turn on once a small temperature drop (0.5°C) is 

detected by the thermistor. This mode uses the heat pump exclusively unless the air 

temperature is outside the operating bounds, defined by the manufacturer as 7°C to 49°C. 

The heating elements will turn on if icing on the evaporator coils is imminent. This mode 

is very efficient, but tank recovery is slow due to the smaller heating capacity of the 

compressor. 

2. Hybrid Mode: Similar to eHeat mode, the heat pump will turn on once a small drop in 

temperature is measured by the thermistor. A more significant drop in temperature (10°C) 

will cause a heating element to turn on. The lower heating element turns on first for 

moderately large draws and the upper element turns on first for very large draws. If the 

lower element turns on first, it will heat the tank to the set point temperature without ever 

using the upper element. If the upper element turns on first, it will remain on until the 

thermistor at the top of tank reads a temperature 3°C below set point temperature. The 

lower element will then turn on to heat the rest of the tank to the set point temperature. 

During our test, once an electric element has turned on, the remainder of the heating 

cycle will be accomplished with electric heat. However, according to the manufacturer, a 
21 

sequence of smaller draws can trigger the use of the upper element but then revert to the 

heat pump after the top of the tank is at set point. This behavior was not seen during our 

tests. 

3. High Demand Mode: This mode is very similar to Hybrid mode, but the electric elements 

will turn on sooner than in Hybrid mode, when the temperature at the thermistor drops 

3°C. 

4. Electric Only Mode: The upper element will turn on after a minimal drop in temperature 

(~0.5°C) at the thermistor and will heat until the top of the tank is 3°C below the set 

point. The lower element will then turn on to heat the rest of the tank. The lower element 

will turn off when the thermistor near the top of the tank reads the set point temperature.

This mode offers a quick recovery to the set point temperature, but is the least efficient of 

the modes due to its sole reliance on electric resistance elements. 

Draw Profile Tests

Draw Profile 1 was implemented in two segments: the morning segment and the evening 

segment. The results of this test are shown in Figure 15. During the first part, which simulated 

four consecutive morning showers, Unit B was able to maintain ‘hot’ water (>40.5°C) for the 

first two showers but the outlet temperature dropped below that temperature during the third 

shower. While supplying the second set of draws, which simulated evening use, the test article 

was able to maintain ‘hot’ outlet temperature during all but one draw. The evening draw profile 

consumed a lower total volume of hot water over a longer period of time, allowing the water 

heater to keep up with demand better than in the morning draw profile. During both portions of 

Draw Profile 1, the compressor came on first, and when demand was too great, electric resistance 

elements took over for the remainder of the heating cycle. The upper element came on first, until 

the top of the tank was slightly below the set point temperature, then the lower element turned on 

to bring the entire tank to the set point temperature. The resulting efficiency was much lower 

than would be accomplished by running the heat pump alone. 
22 

Figure 15. Draw Profile 1 for Unit B

Draw Profile 2 consisted of many short draws spaced over a 6-hour period. The results of this 

test show that Unit B performed well under this low-demand condition since only the compressor 

was needed to maintain set point temperature. 

COPhp Tests

Results of the COPhp tests are plotted below in Figure 16. As expected, COPhp increases as the 

inlet air temperatures rise and the COPhp decreases as the water temperature in the tank 

increases. All tests were run in the eHeat operating mode to achieve compressor-only operation. 

At the lowest inlet air temperatures, operation switched from the heat pump to the upper element 

while the average tank temperature was still colder. Under normal operation, the heat pump 

would not turn back during the heating cycle, so power was manually cycled. This eventually 

allowed the heat pump to operate continuously to complete the heating cycle. Icing may have 

been the reason that the upper element was called for. 
23 

Figure 16. COPhp traces for Unit B

The two warmest COPhp tests were repeated with the same inlet dry bulb temperature for high 

and low relative humidity. It was expected that the inlet conditions with the higher humidity (i.e. 

higher wet bulb temperatures) would perform better than the lower humidity conditions. Figure 

17 shows this comparison for the inlet dry bulb condition at 35°C with the relative humidity at 

40% and 19%. The corresponding wet bulb temperatures for these conditions are 24°C and 17°C, 

respectively. The results show that the performance improvements were not significant for the 

COPhp test run at the higher inlet wet bulb temperature. On average, the increase in COPhp was 

~1%. The tank recovery time for two humidity levels is shown in Figure 18. The lower humidity 

slowed the recovery rate from 9.6°C/hr to 9.4°C/hr. These results show that humidity has a 

small, but not insignificant, impact on the performance of this unit. 
24 

Figure 17. Effect of humidity on COPhp for Unit B Figure 18. Effect of humidity on recovery time 

for Unit B

Reduced Airflow Tests

Blocking the airflow path by one-third resulted in a minor system performance drop, and an 

additional small performance reduction resulted from the two-thirds airflow blockage. However, 

in neither case was performance impacted significantly; this can be attributed to the variable 

speed fans. These results are shows in Figure 19. For the case of the one-third airflow blockage, 

the fan speed was seen to be greater than the nominal case at the higher water temperatures only.

In the case of one-third blockage, the fan power reached a maximum of 23 W, the same 

maximum of the unblocked case, but it remained at this maximum value for 1-1⁄2 hour longer 

before the fan speed decreased as the tank temperature approached set point. In contrast, for the 

two-thirds airflow reduction, the fans immediately went to their maximum speed, corresponding 

to 28 W, and remained there for the entire test in an apparent effort to counteract the airflow 

restriction.
25 

Figure 19. COPhp traces for Unit B with reduced airflow

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit B: 

1. Slow recovery time. The recovery time for this unit is very long if the heat pump alone is 

used. This could be an issue for homeowners who demand equivalent performance and 

improved efficiency when switching to HPWH technology. A smaller upper electric 

resistance element that could be used in conjunction with the compressor would help 

improve recovery time while still taking advantage of the efficiency of the heat pump. A 

larger tank would not improve recovery time but would provide a larger buffer. Even in 

situation of high demand, a large tank could consistently supply hot water without 

needing to revert to the electric elements.

2. Control algorithm did not switch back to the heat pump if electric resistance 

elements are called for. In Hybrid and High-Demand modes, if the electric resistance 

elements are used because of a high-demand situation, the compressor did not turn on 

again until the reheat cycle is complete. According to the manufacturer, the heat pump 

should turn on after the upper element in times of moderate to high demand, but that 

behavior was not reproduced in the laboratory. However, the manufacturer regularly sees 

this behavior in the numerous field installations they monitor. The control logic that 

allows the heat pump to turn back on may need to be revised so that it is easier to 

achieve, as opposed to the logic that uses the upper element and then the lower element 

when demand is high. 
26 

3. User interface is not user-friendly. A simple task like changing the operating mode 

requires navigating through multiple screens. A more straightforward display may 

encourage people to maximize the efficiency of the unit by changing the temperature 

setting and operating mode to meet their changing needs. 
27 

NREL/PIX 18667, 18672, and 18669Credit: Kate Hudon 

Unit C HPWH Summary

Overall Impressions and Observations

Unit C is the most complicated of all the units tested, in both physical design and control logic. 

To transfer heat from the heat pump to the water, Unit C pumps water from the bottom of the 

tank, through a coaxial heat exchanger/condenser at the top of the unit, and back into the top of 

the tank. This requires a pump that consumes 70 W any time the heat pump is running. The 

pump also thoroughly mixes the tank, which can lead to a decrease in outlet temperature since 

the relatively cold inlet water is quickly mixed with the hot water in the tank. Unit C did not 

operate its compressor over the range of operating temperatures stated in its manual; this 

behavior is described in more detail below. However, when in the optimal air conditions, the 

high capacity compressor can quickly heat a full tank of water. This unit also has two smaller (2 

kW each) electric resistance elements, one of which can operate concurrently with the heat 

pump. This allows for faster and more efficient tank recovery than if a larger electric element 

was used alone. Unit C performed very well in a narrow range of conditions but suffered 

performance impacts outside that range. 

Installation and Usability Considerations

The inlet and outlet water lines are located on the side of the tank. The air filter resides on top of 

the unit and is easily accessible for the homeowner. Access to the heat pump components for 

maintenance and repair can be achieved by disassembling the heat pump casing. 

Figure 20. Unit C in the test plenum (left), the compressor and coaxial heat exchanger (top right), 

and the water circulation pump, evaporator, and control wiring (bottom right)

Compressor

Coaxial 

Condenser

Pump

Control Wiring

Evaporator
28 

The air enters this unit from the top and is pushed across evaporator coils that encircle about 

two-thirds of the area around the heat pump. A minimum distance between the air inlet and the 

ceiling is specified by the manufacturer, and the location of the water heater must be considered 

to ensure installation requirements are met. This unit can currently be purchased at Home Depot. 

The user interface is straightforward and user-friendly. This interface is used to make changes to 

the set point temperature and to select an operating mode. This unit can be operated in three 

modes: Energy Saver, Normal, and Electric Only. The interface provides a set point gradient 

from Hot to Normal to Vacation, without providing actual temperature of each set point. The 

homeowner must reference the manual to learn what setting corresponds to what temperature. If 

the compressor is turned on, there is a 20-minute delay before the compressor and fan turn on. 

This occurs both during initial start-up and if the user switches to Energy Saver mode from 

Electric-Only mode. In Electric-Only mode, the resistance heaters turn on immediately.

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit C HPWH and provide 

observations made during testing. 

Operating Mode Tests

The heat pump and upper heating element in this unit can operate concurrently. Both heating 

elements can also operate at the same time. A thermistor located near the lower heating element

is used to trigger the operation of the heat pump and/or the electric resistance elements. A second 

thermistor located near the upper heating element determines when the heat pump or heating 

elements should be turned off. If the tank is filled with water below 27°C at initial start-up, the 

heat pump will not run and the upper resistance element will be used until the temperature 

exceeds 27°C. The heat pump will then turn on, if allowed by the operating mode. This 

information was provided by the manufacturer and confirmed during testing. 

Operation Modes and Description:

1. Energy Saver Mode: The heat pump turns on when the thermistor located near the lower 

heating element reads a temperature around 22°C. The heat pump operates exclusively if 

the water temperature set point is 52°C or below, unless the air temperature is outside of 

the manufacturer-stated operating bounds (4°C and 49°C). When the set point is at its 

highest (58°C), the heat pump and upper element are used primarily. The use of the heat 

pump alone versus the use of the heat pump and upper element appears to be tied to the 

set point temperature, rather than draw size. Also, other than times when the air 

temperature was outside of acceptable bounds, there does not appear to be a scenario in 

Energy Saver mode that cause the two electric elements to operate alone, without the heat 

pump. 

• Even though the user manual states that the heat pump will operate in air 

temperatures between 4°C and 49°C, we did not see continuous operation of the heat 

pump for dry bulb temperatures below 14°C or above 35°C. For air temperatures 

below 14°C, ice built up on the evaporator coils, as indicated by three surface 

mounted thermcouples that were installed on the coils at the inlet, middle, and exit of 

the evaporator for the purposes of testing. This caused the heat pump to cycle on and 

off three times before switching to electric resistance heat for the remainder of the 
29 

heating cycle. Above 35°C, the heat pump cycled on once before switching to the 

heating elements.

• It should also be noted that the upper element turns on at the very end of the heating 

cycle for the higher set points. (See Anomalies section below for further discussion.)

2. Normal Mode: This mode is very similar to the Energy Saver mode. For a 49°C tank set 

point, the heat pump will turn on alone, except at the very end of the reheat cycle when 

the upper heating element will also be used. For the highest set point (58°C), the heat 

pump and upper electric element turn on. The temperature trigger for the heat pump is the 

same as in Energy Saver mode. There is no apparent advantage to this operating mode 

when compared to the Energy Saver mode. 

3. Electric-Only Mode: Both heating elements will turn on when the thermistor temperature 

drops to 20°C but the lower element turns on first, after a small drop in temperature 

(~0.5°C) is detected by the lower thermistor. The upper element will turn off when the 

top of the tank has reached set point and the lower element will remain on to finish 

heating the lower half of the tank. 

Draw Profile Tests

Draw Profile 1 was tested in two segments, the morning segment and evening segment. During 

the morning segment, which simulated four consecutive morning showers, Unit C was able to

maintain acceptably hot outlet water (>40.5°C) for the first shower but the outlet temperature 

started to drop by the beginning of the second shower. The outlet water temperature dropped 

quickly because the circulation pump turns on with the compressor and mixes the cold water 

from the bottom of the tank with the hot water at the top of the tank. During this portion of the 

draw profile, the heat pump cycled on and off three times, alternating between the heat pump and 

both electric resistance elements, before the elements remained on for the remainder of the 

heating cycle.
30 

Figure 21. Draw Profile 1 for Unit C

During the second part of Draw Profile 1, which simulated evening use, Unit C was able to 

maintain hot outlet temperature during all draws. The compressor turned on twice during this 

profile, and the upper element assisted the compressor during a portion of the test. The combined 

use of the heat pump and electric resistance elements during Draw Profile 1 resulted in lower 

energy efficiency compared to running the heat pump alone. 

Draw Profile 2 consisted of many small draws over a 6-hour period. Unit C performed well 

under this low-demand situation since only the compressor was needed to maintain set point 

temperature. 

COPhp Tests

The results of the COPhp tests are plotted below in Figure 22. As expected, COPhp increases as 

the inlet air temperatures rise and the COPhp decreases as the water temperature in the tank 

increases. For inlet dry bulb temperatures ranging from 25°C to 35°C, the heat pump heated the 

water quickly and efficiently. For air temperatures below 25°C (16°C wet bulb), icing occurred 

on the evaporator coils during a portion of the heating cycle, resulting in compressor cycling. For 

the COPhp tests at these cooler inlet air conditions, the power was reset after the compressor 

cycled three times to force heat pump operation until the set point was reached. For the case of 

20°C inlet air, the compressor cycled at the lower water temperatures but remained on once the 

average tank temperature reached ~24°C. Below 20°C dry bulb (14°C wet bulb), the heat pump 

was never able to achieve continuous operation. 
31 

For COPhp tests at air temperatures at and above 38°C, the compressor cycled on and off once 

before switching to both electric elements but did not turn on again. This was due to a fault that 

occurs when the temperature difference across the compressor is less than 20°C. This fault 

prevented the compressor from running at both high and low humidity for dry bulb temperatures 

above 35°C. It is unclear what this control feature is trying to prevent.

Figure 22. COPhp traces for Unit C

The COPhp test run at the highest dry bulb temperature was repeated with a lower relative 

humidity to determine the impact of humidity on performance. Figure 23 shows this comparison 

for the inlet dry bulb condition at 35°C with the relative humidity at 40% and 19%. The 

corresponding wet bulb temperatures for these conditions are 24°C and 17°C, respectively. 

The COPhp dropped by ~20% at the lower humidity case, showing that the performance of Unit 

C is sensitive to the humidity of the ambient air around it. The tank recovery time is shown in 

Figure 24. This shows that the higher humidity increased recovery rate from 14.1°C/hr to 

17.8°C/hr relative to the lower humidity case at the same dry bulb temperature. 
32 

Figure 23. Effect of humidity on COPhp for Unit C Figure 24. Effect of humidity on recovery time 

for Unit C

NOTE 1: Because of control logic restrictions, when initially turned on, the compressor does not 

turn on for water temperatures lower than 27°C. In order to perform the COPhp tests with 3°C 

starting water temperatures, we added a circuit containing a potentiometer in parallel with the 

lower thermistor. Using a manual switch, we could provide the control board with the actual 

water temperature or a false, warmer temperature. This successfully allowed the compressor to 

run for the coldest water temperatures. However, icing occurs more often on the evaporator when 

the water was cold (<27°C), which explains why that control logic is built in. 

NOTE 2: Unit C regularly uses its electric resistance elements when in its most efficient mode. 

Manual switches were installed on both elements that could be used to disable their use. These 

switches were only used during the COPhp tests to ensure that only the compressor was running. 

NOTE 3: To determine why the compressor was not operating under various conditions, we 

looked at the Fault Indicator Light, which is a red LED located on the circuit board (behind the 

control panel). This light will blink a prescribed number of times to indicate the reason for a 

compressor fault. The LED is not visible unless a hole is drilled in the plastic housing. The 

manufacturer provided us with flash code definitions to help diagnose problems. 

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 25. The results of the one-third 

reduced airflow test show a slight, but insignificant, decrease in performance. The results of the 

two-third reduced airflow test show a more significant performance reduction, ranging from 5% 

to 20% over the course of the heating cycle. 
33 

Figure 25. COPhp traces for Unit C with reduced airflow

Anomalies

The heat pump design appears to be optimized for speed at the expense of the operating range. 

The compressor is oversized, which results in fast recovery times, but also contributes to a more 

limited ambient temperature operating range. For air temperatures below ~15°C, icing starts to 

accumulate on the evaporator coils and the compressor will cycle on three times before switching 

over to the electric resistance elements to heat the water. In addition, when air temperatures are 

at or above ~38°C, the heat pump will cycle on once and then turn off in favor of the electric 

resistance elements. As mentioned previously, this is due to control logic that shuts off the heat 

pump if the temperature difference across the compressor is less than 20°C. (The reason for this 

restriction is still unknown and may or may not be a result of the oversized compressor.) These 

restrictions greatly limit the operating environments where this HPWH will be an efficient water 

heating option. In particular, garages and other unconditioned (or passively-conditioned) spaces 

may be poor locations for this unit. 

Another result of the heat pump being designed for speed is that the upper resistance element 

must come on in place of the heat pump to finish the heating cycle. The refrigerant used in Unit 

C’s heat pump, R-410a, operates at a higher pressure than R-134a. At the higher refrigerant 

temperature, this pressure starts to approach the maximum capacity of the copper tubing in the 

heat pump. Design changes could prevent the need for the electric resistance elements at the 

higher tank temperatures, such as reducing the diameter of the copper tubing, but these changes 

may slow the recovery time for the system. 
34 

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit C: 

1. Condenser design is ineffective. The circulation pump that brings water to the coaxial 

heat exchanger at the top of the unit was found to be a detriment to the outlet 

temperature. For example, when performing water draws, the circulation pump mixes the 

cold water at the bottom of the tank with the hot water at the top of the tank, limiting 

availability of water at the desired (a.k.a. set point) temperature. It is assumed that the 

pump is needed to achieve sufficient (forced) convection in the coaxial condenser and 

prevent the water from boiling.

2. Inefficient circulation pump. The circulation pump draws ~70 W and produces enough 

heat to burn someone who touches it. This wastes energy and is a safety risk if the outside 

cover that protects the circulation pump is ever removed. If the pump could be moved to 

inside the heat pump compartment, this would reduce the safety risk and allow the heat 

pump to reabsorb the heat generated by the circulation pump.

3. Control logic consists of long delay before compressor starts up. The compressor in 

this unit has a starting delay of 20 minutes, presumably for internal diagnostics. This does 

not seem to be necessary. The long delay also might make homeowners think that 

something is wrong with the heat pump and switch its operating mode to all resistance 

heat to make sure that water will be heated. 

4. Control logic restricts upper range of operating conditions. The heat pump will not 

run at air temperatures above ~38°C. Since this range also represents the range in which 

the heat pump could be most effective, the control logic should be reconsidered so the 

heat pump can operate at higher air temperatures. It is also unclear why this restriction 

was implemented by the manufacturer, from either a safety or performance perspective.

5. Excessive icing occurs on the evaporator coils at moderate air temperatures. 

Frequent icing occurs at air temperatures less than 20°C, making this unit an unfavorable 

choice for unheated or passively heated installation locations in cooler climates. This 

appears to be a result of using a larger compressor to speed recovery time.

6. The upper resistance element is needed to achieve set points above 49°C. As 

mentioned above, the design of the heat pump necessitates the use of the upper element at 

higher tank temperatures (and thus higher refrigerant temperatures) to avoid reaching the 

maximum pressure of the copper tubing. 
35 

NREL/PIX 18676, 19460, and 19461Credit: Kate Hudon

Unit D HPWH Summary

Overall Impressions and Observations

Unit D is a simple HPWH with only one operating mode and no control panel. Where other 

companies have created many options and interactive displays, Unit D’s manufacturer took the 

opposite path. There is only one operating mode and no way to turn the unit off. The controls for 

changing set point temperature are difficult to access, making it unlikely that users will ever 

adjust the default setting. There is a single, small electric resistance heater located near the hot 

water outlet, designed to provide a small amount of boost heating but the resistance heater alone 

could not be used to heat the entire tank. The large tank can provide plenty of hot water, but the 

heat pump and back up element are small, and so recovery time is slow. However, this unit has a 

high energy factor (EF) rating and so can provide very efficient water heating. 

Installation and Usability Considerations

Unit D’s inlet and outlet water lines are located on the side of the tank. The 80 gallon unit tested 

was just over 6 ft tall. The air enters the unit from the front, is pulled across the evaporator coils 

located in the middle of the heat pump, and exits out the back of the unit. There is no air filter in 

this unit. Currently, Unit D is not available through a major retailer, but can be purchased 

through plumbing suppliers. 

Figure 26. Unit D, an 80 gallon HPWH, installed in the test plenum (left). A close up of 

the air intake (top right) and the heat pump (bottom right) are also shown.
36 

There are no operating modes for Unit D and it is intended to be hard wired when installed, 

meaning that the user cannot manually turn the unit on or off. There are knobs located inside on 

the control board that can be used to change the temperature set point for the heat pump and 

electric resistance heater separately. However, the top of the heat pump case must be removed to 

access these knobs and there are no labels on the knobs to indicate the temperature range 

available or to identify which heating source they control. For laboratory testing, Unit D was 

wired to a switch for power and, when switched on, the fan and compressor turned on 

immediately. Since the default set point for the heat pump is 60°C and it is unlikely that the set 

point will be changed due to the inaccessibility of the controls, some tests were done with this set 

point to capture the more likely operating conditions. 

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit D and the observations made 

during testing. 

Operating Mode Tests

Unit D has a single thermistor as a control input, located at the top of the tank. There is only one 

operating mode for this unit. The heat pump can operate at the same time as the electric 

resistance element, but the electric element cannot run on its own. There are two separate set 

points for the heat pump and resistance element. There are knobs to adjust these set points inside 

the heat pump casing, on the control board. Based on our tests, the compressor set point can vary 

between 54°C and 60°C and the resistance heater set point temperature has a range from 29°C 

and 57°C. There are no markings to indicate the current set point for either heat source. 

Operation Mode description:

1. On: The heat pump will turn on when the thermistor registers a small drop in temperature 

(2°C) from the heat pump set point. The heating element will turn on when the top of the 

tank drops 5°C from the element set point. When the draw is large enough to trigger the 

heating element, it will remain on until the thermistor reaches the heating element set 

point. The default element set point is 55°C, which is below the default heat pump set 

point of 60°C. The rest of the tank will be heated with the heat pump until the thermistor 

reading reaches the heat pump set point. According to the user manual, the operating 

range for this water heater is between air temperatures of 6°C and 42°C. 

Draw Profile Tests

Draw Profile 1 was tested in two parts. Unlike the other units where the draw profile tests were 

done with a 49°C set point, the set point for Unit D cannot be set below 54°C and it is unlikely 

that users will ever adjust the default set point of 60°C, so the draw profiles were run at the 

default set point temperature. During the morning portion of Draw Profile 1, four showers are 

simulated over the course of a little more than 1per hour. The outlet temperature through the first 

three remained close to 60°C but started to decrease during the fourth shower, with the outlet 

temperature falling to a minimum of 53°C. The heat pump turned on during the third shower and 

the electric resistance element turned on about an hour after the fourth shower was complete. The 

resistance element remained on for a half an hour, after which, the heat pump finished heating 

the tank to set point. 
37 

The evening draws produced outlet water at 60°C for the first half of the draws, but the outlet 

temperature started to drop by the end of the evening segment. The final draw delivered water at 

54°C. The heat pump came on a little past the halfway point of the evening draw profile and 

remained on until set point was reached again. 

Figure 27. Draw Profile 1 for Unit D

Draw Profile 2 consisted of many small draws over a 6-hour period. Unit D performed well for 

this low-demand situation. The compressor turned on to reheat the tank about 3-1⁄2 hours into the 

6-hour profile. 

COPhp Tests

The results of the COPhp tests are plotted below in Figure 28. As expected, COPhp increases as 

the inlet air temperatures rise and the COPhp decreases as the water temperature in the tank 

increases. The electric resistance heater was disabled during these tests to ensure that it would 

not turn on. Even though COPhp decreased for lower air temperatures, Unit D maintained a 

consistently high level of performance for all air conditions tested. 
38 

Figure 28. COPhp traces for Unit D 

The two warmest COPhp tests were repeated with the same inlet dry bulb temperature for high 

and low relative humidity. Figure 29 shows this comparison for the inlet dry bulb condition at 

35°C with the relative humidity at 40% and 19%. The corresponding wet bulb temperatures for 

these conditions are 24°C and 17°C, respectively. 

The results in Figure 29 show that the performance increases were small for the COPhp test run 

at the higher inlet wet bulb temperature. For the lower tank temperatures, a difference in COPhp

of approximately 5% can be seen. As the tank temperature increases, the performance difference 

drops to about 2%. The tank recovery time is shown in Figure 30. A reduction in recovery rate 

from 8.1°C/hr to 7.7°C/hr was observed in the lower humidity case. These results show that 

humidity has a small impact on the performance of this unit. 
39 

Figure 29. Effect of humidity on COPhp for Unit D Figure 30. Effect of humidity on recovery time 

for Unit D

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 31. The results of the one-third 

airflow blockage test showed an insignificant decrease in performance. The results of the twothirds airflow blockage test showed a slight decrease in performance, ranging from ~3% to ~4 % 

over the course of the heating cycle. 

Figure 31. COPhp traces for the Unit D with reduced airflow
40 

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit D: 

1. The 80 gallon unit is tall. The large tank capacity has many benefits from a performance 

perspective, but its physical size may limit the homes in which it can be installed. Unit D

is over 6 ft tall and it could be limiting depending on where the hot water heater is 

installed in the home. 

2. Tank recovery does not start fast enough. While the large tank provides ample hot 

water, reheating the tank is very slow. Rather than having the heat pump turn on after a 

small or moderate volume draw, Unit D waits to turn on both the heat pump and electric 

resistance heater until almost all the hot water in the tank has been drawn. This will result 

in very long recovery times during which no hot water will be available to the 

homeowners. 

3. There is only one operating mode. Unit D has only one operating mode. Users may see 

this as a disadvantage since they cannot change modes in anticipation of higher hot water 

demand (having guests, for instance). The advantage to having only one operating mode 

is that users will be unable to switch to using all resistance heat by accident, but the lack 

of options will likely be seen as a disadvantage from the consumer’s perspective. 

4. Means of controlling set point temperatures are inaccessible. The knobs used to 

control heat pump and resistance heater set point temperatures are only accessible by 

taking off the top of the water heater casing. Even then, the knobs are not very obvious 

and there are no markings to indicate current set point and available range of set points. 

The default values will likely never be changed, even though the heat pump set point is 

60°C, which is higher than the recommended set point temperature. 
41 

NREL/PIX 18919, 19462, and 19463Credit: Kate Hudon

Unit E HPWH Summary

Overall Impressions and Observations

Unit E uses immersed condensing coils to reduce recovery time and provides an option to duct 

the outlet air. The heat pump operates over a wide ambient temperature range and has a defrost 

feature that allows it to operate in very cold air temperatures without reverting to the electric 

resistance element. However, the user interface is difficult to operate and the air filter is nearly 

inaccessible. Unit E also provides an option to duct the outlet air, which adds a degree of 

flexibility. Taking advantage of the outlet duct option would add to installation costs, but sending 

the cold outlet air out of the house or to the existing HVAC ducts in the house could add value to 

the water heater installation and eliminate some of the negative impact on the home’s heating 

system in colder climates. A preproduction model of the Unit E HPWH was provided by the 

manufacturer for these tests and as such, some features and functionality will be modified in the 

full production model. 

Installation and Usability Considerations

Unit E’s inlet and outlet water lines are located on the side of the tank. The 66 gallon unit that 

was tested is nearly 6 ft tall. There also is a 50 gallon version of Unit E that is a little less than 5

ft tall. The 66 gallon unit tested requires an airflow rate of about 300 cfm. The air enters the unit 

from the right (when viewed from the front), is pulled across the evaporator coils located in the 

Figure 32. Unit E installed in test plenum (left). Some of the refrigerant system is shown 

(upper right), along with the blower that is designed to connect to a 6 in. duct (lower 

right)
42 

middle of the heat pump, and exits out the top of the unit on the left side. This unit is designed to 

allow the outlet air to be ducted so the unit has a blower instead of a typical axial fan. Ducting 

the outlet air would be useful in both hot and colder climates. If installed in a home in a hot 

climate that has a central air conditioning system, a duct could be used to route the cold outlet air 

from the HPWH to the existing duct system in the house. Additionally, in colder climates, the 

duct could be used to send the outlet air outside, so as not to add to the heating load during the 

winter time. This would add to the cost and complexity of installation but it could improve the 

HPWH’s impact on the HVAC system. The air filter is only accessible if the heat pump cover is 

removed. If the outlet was ducted, it would be impossible to remove the heat pump cover without 

disassembling the duct. Even if the ducting option was not in use, the heat pump cover is not 

easily removed and power must be disconnected, making it very unlikely that any users will ever 

clean the air filter. Currently, Unit E is not available through a major retailer.

The user interface displays temperature settings, as well as current air and water temperature 

readings, and can be used to change a number of options. There are three operating modes: Econ 

(heat pump only), Auto (combination heat pump and electric resistance heat), and Heater (all 

electric resistance heating). There are indicators on the user interface to show when the unit is 

actively heating and what heat source(s) are being used. However, the usability of the interface is 

poor, making it difficult to perform simple tasks like changing the set point temperature. One 

unique control feature is that the user can set time periods that restrict when the heat pump will

operate in Econ mode. This may be a nice feature for advanced users, but there is no default time 

period, so if Econ mode is selected and operating periods have not been defined, the heat pump 

will never turn on. This feature may discourage users from using the most efficient mode. 

This unit contains two 4 kW electric resistance heaters but only one of them will be used during 

normal operation. The second resistance heater is wired for power separately from the main 

power supply, so it will never turn on in typical installations. According to the manufacturer, the 

second resistance heater should only be used if the primary electric heater fails. The backup 

resistance heater sits just below the midpoint of the tank and the primary resistance heater sits 

above that, a third of the height of the tank from the top. 

Qualitative Test Results and Observations

The descriptions below explain the results of the tests run on Unit E and the observations made 

during testing. 

Operating Mode Tests

Unit E has two thermistors that are monitored to control the heat pump and electric element. The 

lower thermistor is located just above the unused back up resistance heater, at roughly the midpoint of the tank. The upper thermistor sits above the usable resistance heater and is halfway 

between the top of the tank and the lower thermistor. The lower thermistor is used to control the 

heat pump and the upper thermistor controls the resistance element. The resistance element and 

heat pump can operate simultaneously. 

Operation Modes and description:

1. Econ Mode: The heat pump will turn on when the lower thermistor measures a 

temperature of 10°C–15°C below the set point. The heat pump will turn off when the 
43 

lower thermistor returns to the set point. The deadband can be adjusted by the user 

through the user interface. According to the manufacturer, the default deadband is 5°C 

but heat pump operation was triggered by larger temperature drops during laboratory 

tests. At a set point of 49°C, a 10°C drop in temperature at the lower thermistor caused 

the heat pump to turn, while at the 57°C set point, a 15°C drop was required for the heat 

pump to turn on. Even though a relatively large draw is required to trigger the heat pump, 

the heating capacity of the heat pump is large enough to recover in a short amount of 

time. 

• A timer period must be defined by the user for the heat pump to turn on at all in 

this mode. However, if one or more timer period is set, the heat pump operates 

well across the operating range. 

• If the air temperature is cold enough to cause icing on the coils while the heat 

pump is operating, a defrost cycle will occur. The fan is turned off for about 2 

minutes while the compressor remains on and the refrigerant reversing valve 

reheats the evaporator coil (now the condenser) to melt the ice. This process is 

repeated as needed until set point has been reached or until icing is no longer a 

problem. 

2. Auto Mode: As with Econ mode, the heat pump turns on once the lower thermistor 

measures a temperature drop from set point of 10°C–15°C. A larger draw will trigger the 

electric resistance heater. The resistance heater will turn on when the upper thermistor 

measures a drop in temperature of 10°C–20°C and will turn off when the upper 

thermistor is at set point. The resistance heater will operate at the same time as the heat 

pump. If the resistance heater is called for, the heat pump will remain on and will 

continue to heat the tank until the lower thermistor reaches set point. Unlike Econ mode, 

a timer period does not need to be set for the heat pump to operate. 

3. Heater Mode: This mode will only use the resistance heating element to heat the tank. 

Similar to the control of the resistance element in Auto mode, the resistance heater will 

turn on when a drop in temperature of 10°C–20°C degrees is detected by the upper 

thermistor. The electric element is located in the upper third of the tank so, in Heater 

mode, only the top third of the tank will be heated. This mode will not satisfy hot water 

demands for most users and is the least efficient of all modes. Heater mode is only 

intended to be used when the heat pump is not functional. 

Draw Profile Tests

Draw Profile 1 was tested in two parts. During the morning segment of Draw Profile 1, four 

showers are simulated over the course of a little more than an hour. Unit E unit was able to 

maintain an outlet temperature of 49°C during the first three showers, but dropped below 40.5°C 

during the fourth shower. By the end of the fourth shower, the outlet temperature had dropped to 

about 38°C. The heat pump turned on during the second draw and remained on for the rest of the 

morning segment until set point was reached. The electric resistance heating element turned on 

during the third shower when average tank temperature started to drop. The evening draws were 

completed with no significant dip in outlet temperature. The heat pump turned on about halfway 

through the evening portion and remained on until the set point was achieved after the profile 
44 

finished. The upper resistance element also turned on for a brief time following the next to last 

draw.

Figure 33. Draw Profile 1 for Unit E

Draw Profile 2 did not provide any additional insight about the performance of the previous 

HPWHs tested, so it was omitted from Unit E’s test schedule.

COPhp Tests

The results of the COPhp tests are plotted below in Figure 34. As expected, COPhp increases as 

the inlet air temperatures increases and the COPhp decreases as the water temperature in the tank 

increases. These tests were run in Econ mode to ensure heat pump-only operation. At the lowest 

air temperatures, the defrost cycle was needed to prevent icing on the coils. During the defrost 

cycle, the fan is turned off for 2 minutes while the compressor remained on. The periods of 

defrost can be seen in dips in the lowest wet bulb temperature COPhp curves. 

An additional cold air test was partially completed at 4.5°C dry bulb and 3°C wet bulb. Since this 

test article claimed better cold weather performance than all the others, this additional test point 

was added. By the time the tank temperature reached 27°C, it became impossible to maintain the 

air conditions due to laboratory limitations. The test did confirm that this unit can operate below 

7°C, the lower limit for ambient temperature for all of the other units, even though repeated 

defrost cycles were required.
45 

Figure 34. COPhp traces for Unit E

The COPhp test run at the highest dry bulb temperature was repeated with a lower relative 

humidity to determine the impact of humidity on performance. Figure 35 shows this comparison 

for the inlet dry bulb condition at 35°C with the relative humidity at 40% and 19%. The 

corresponding wet bulb temperatures for these conditions are 24°C and 17°C, respectively. There 

was a significant performance difference between the dry and humid air conditions, especially at 

the lower tank temperatures. The tank recovery time is shown in Figure 36. This shows that the 

higher humidity case resulted in an increase in recovery rate from 10.8°C/hr to 11.8°C/hr. 
46 

Figure 35. Effect of humidity on COPhp for Unit E Figure 36. Effect of humidity on recovery time 

for Unit E

Reduced Airflow Tests

The results of the reduced airflow tests are shown in Figure 37. Since the four previous units 

tested showed little to no impact when one-third of the inlet area was blocked, that test was 

omitted for Unit E. The two-thirds airflow blockage test resulted in no noticeable performance 

degradation. 

Figure 37. COPhp traces Unit E with reduced airflow
47 

System Drawbacks

Below is a list of system drawbacks that could be improved upon to enhance the capabilities of 

Unit E. It should be reiterated that the unit tested was a preproduction model and some 

functionality is expected to be modified in the full production version. 

1. User interface is difficult to use. Changing operating modes is straightforward since 

each mode has its own button, but any other action, including simple tasks like changing 

the set point temperature, are impossible without the manual. 

2. Internal condensing coils do not heat the bottom of the tank. The bottom of the 

condensing coils sits about 8 in. above the bottom or the tank, meaning that about 8 

gallons of water at the bottom of the tank remain cold even when the heat pump is on. 

This effectively reduces the tank capacity from 63 gallons (as measured) down to 55 

gallons. 

3. Backup resistance element is too high in the tank. The backup electric resistance 

element sits about one third of the tank height from the top of the tank. When in Heater 

mode, the resistance heater can only heat the top third of the tank. Heater mode is not 

intended to be used unless the heat pump malfunctions, but if that was to happen, this 

water heater may not be able to provide sufficient hot water. 

4. Timer feature will likely cause more harm than good. Up to three time periods can be 

set for heat pump operation when in Econ mode. If the heat pump is called for at a time 

outside of these ranges, it will not turn on. This feature is designed to allow homeowners 

to control when cold air is generated by their water heater. However, there is not a default 

timer period, so if the user tries to use Econ mode without setting a time period, nothing 

will happen. 

5. Extra resistance heating element is unnecessary. Unit E comes with a second, back-up 

resistance heating element that is only used if the main resistance heater fails. In fact, the 

second resistance heater has a separate power supply that must be wired separately for 

use. This seems like an unnecessary feature that just increases its cost. Also, the back-up 

resistance element sits lower in the tank than the main resistance element. If it was 

eliminated, the main resistance element could sit lower, allowing it to heat a larger 

portion of the tank. 

6. Air filter is inaccessible. The air filter is located under the heat pump casing and cannot 

be accessed without fully disassembling the case and the outlet ducting (if in use). Users 

will likely never clean the air filter unless it is easily accessible. 
48 

Data Comparison and Analysis

Operating Mode Control Comparison 

The following table summarizes the control logic for each operating mode for each test article. 

More detailed information about each unit’s control logic can be found in the specific water 

heater sections, but the tabular format allows for quick comparison between different units.

Table 3. Comparison of Operating Modes and Control Logic for All Five Units

Unit A Unit B Unit C Unit D Unit E

Most-efficient mode: Efficiency eHeat EnergySaver On Econ

Heat pump? Yes Yes Yes Yes Yes

Control logic to turn 

on heat pump:

ΔTtank,mfr =

-5°C1

ΔTupper =

-0.5°C Tlower = 22°C ΔTupper =-2°C

ΔTlower =

-10°C

Resistance elements? No No Yes Yes No

Control logic to turn 

on heating elements: - - Tlower = 22°C2

ΔTupper =

-10°C3 - 

Hybrid Mode: Hybrid Hybrid Normal N/A Auto

Heat pump? Yes Yes Yes - Yes

Control logic to turn 

on heat pump:

ΔTtank,mfr

= -5°C

ΔTupper

= -0.5°C

Tlower

= 22°C - ΔTlower =-10°C

Resistance elements? Yes Yes Yes - Yes

Control logic to turn 

on heating elements:

ΔTtank,mfr

= -10°C5

ΔTupper

= -10°C4 Tlower = 22°C - ΔTupper =-10°C

Additional Mode: N/A High 

Demand N/A N/A N/A

Heat pump? - Yes - - -

Control logic to turn 

on heat pump: - ΔTupper= -0.5°C - - - 

Resistance elements? - Yes - - -

Control logic to turn 

on heating elements: - ΔTupper= -3°C - - - 

Resistance-only 

Mode:

Electric 

Only

Electric 

Only Electric Only N/A Heater

Heat pump? No No No - No

Control logic to turn 

on heat pump: - - - - - 

Resistance elements? Yes Yes Yes - Yes

Control logic to turn 

on heating elements:

ΔTtank,mfr

= -2°C

ΔTupper =

-0.5°C Tlower = 20°C - ΔTupper =-10°C

Table 3 Notes:

1 Unit A uses following equation for control: 

Ttank,mfr = �3 ∗ Tupper + Tlower� 4⁄

2 The upper heating element will turn on concurrently with heat pump in EnergySaver mode and Normal 

mode if tank set point is ≥54°C. 
49 

3 The heating element and heat pump can run at the same time. Once called for, the heating element will 

remain on until Tupper reaches the heating element set point, which is set separately from the heat pump set 

point.

4 If heating elements are called for in Hybrid or High Demand mode, the heat pump will not turn on again. 

The heating elements will be used exclusively to finish the heating cycle.

5 Upper element and heat pump cannot operate simultaneously. When the controller calls for the upper 

element, the heat pump turns off until Tupper reaches set point. Then the heat pump will turn back on to 

finish heating the tank. 

COPhp Test Results Comparison

The COPhp tests were performed to determine the performance of the HPWH operating with the 

heat pump alone. This allows the units to be compared against each other as they operate under 

various average tank temperatures. Figure 38 shows a comparison of the COPhp curves for the 

five integrated HPWHs for the inlet wet-bulb air temperature of 14°C (67°F). 

Figure 38. COPhp comparison for five HPWHs at 14°C inlet wet bulb air temperature 

This comparison shows that the COPhp curves for Units B and D are very similar, both in shape 

and magnitude. Unit A is also similar to Units B and D in the lower range of average tank 

temperatures, but the performance drops off more significantly at the higher tank temperatures. 

Unit E performed very well at lower average tank temperatures, but the slope of the curve is 

greater, resulting in lower performance than the other units at the higher average tank 

temperatures. It should be noted that the shapes of the curves appear to depend on the type of 

refrigerant used. Units A, B and D use R134-a and their COPhp curves have a linear 

characteristic shape. Units C and E use R410-a and their COPhp curves decay exponentially. 

Another explanation for the similar shapes of Unit C and E is that the heat transfer from the 

refrigerant and the water is more direct, whereas the other three units have to transfer heat 

through the wall of the copper tubing and the wall of the tank before getting to the water. It

should also be noted that a typical range of average tank temperatures for a water heater in a 
50 

home is between 40°C and 50°C. Over this range, Units B and D have the best performance in 

terms of energy efficiency. 

Another key performance consideration is the heating capacity of the heat pump. Figure 39

shows a comparison of the heating capacity for the five integrated HPWHs for the inlet wet-bulb 

air temperature of 14°C. 

Figure 39. Heating capacity comparison for five HPWHs at 14°C inlet wet bulb air temperature 

This comparison shows that the Units B and D have the lowest heating capacity, so although 

they are very efficient at the higher average tank temperatures, they will take longer to reheat 

than the other units. The heating capacity curve for Unit C, much like the COPhp curve, is 

irregular until the average tank temperature reaches about 25°C because the heat pump could not 

sustain operation due to icing at the lowest tank temperatures. 

In the end, a homeowner should weigh energy efficiency with thermal performance. For homes 

with low hot water demand, the high efficiency, low heating capacity units should be able to 

satisfy demand and provide highly efficient water heating. For high use situations, heating 

capacity and tank volume may become more important factors. 

Draw Profile System COPsys Comparison 

COPsys values were calculated for the morning and evening portions of Draw Profile 1 and for 

Draw Profile 2. The purpose of these calculations is to determine realistic performance numbers 

for the units tested. 

As mentioned previously, the morning portion of Draw Profile 1 was designed to represent a 

high demand situation. All test articles, except for Unit A, resorted to electric resistance

supplemental heaters. The evening portion of Draw Profile 1 was designed to represent the 

evening hot water use for a typical household. These draws are spaced further apart than the 

morning draws and therefore represent a medium demand situation. For the smaller tank units, 
51 

Units B, C, and E, the electric resistance elements were needed to supplement the heating 

process. 

Draw Profile 2 represented a low demand situation spanning across 6 hours. None of the HPWHs 

used back-up electric resistance heat to supplement the reheat cycle. Since COPsys can be 

estimated using the COPhp curves for Draw Profile 2, it was decided not to run this profile for 

Unit E. 

Table 4. Draw Profile System COPsys Values

HPWH Mfr. COPsys 

(DP1 – Morning)

COPsys 

(DP1 – Evening) COPsys (DP2)

Unit A 3.55 ± 0.08 3.42 ± 0.08 2.62 ± 0.06

Unit B 1.21 ± 0.03 1.81 ± 0.05 3.46 ± 0.06

Unit C 1.44 ± 0.04 2.61 ± 0.06 2.53 ± 0.05

Unit D 3.85 ± 0.06 5.37 ± 0.05 3.09 ± 0.06

Unit E 1.68 ± 0.05 2.77 ± 0.07 N/A

The results from the Draw Profile COPsys tests are shown in Table 4. It should be noted that 

these tests include the standby losses incurred by the tanks during the test. The morning results 

show that the units with the smallest tanks, Units B and C, had the lowest COPsys values. These 

units had a difficult time keeping up with demand and had to rely extensively on the back-up 

electric resistance heating elements. Unit D had the highest COPsys for the morning draw profile. 

This can be attributed to both its large storage tank and the small heating capacity of its heat 

pump. The other two units performed as expected based on the sizes of their storage tanks. 

For the evening portion of Draw Profile 1, all the HPWHs except Unit E performed better than 

they did in the high demand situation of the morning draws. Unit B has the lowest COPsys due to 

its control logic, which did not switch back to the heat pump once the electric resistance 

elements are enabled, due to the combined size of the draws. Units C and E had modest increases 

in COPsys during the evening portion of Draw Profile 1 because the electric resistance heating 

elements were used less than for the morning draws. Unit A experienced a slight decrease in

COPsys. It is expected that these numbers are similar because Unit A did not need its electric 

resistance heat for either of these profiles. It is slightly greater for the case of the morning draws 

because average tank temperatures dropped lower than in the case of the evening draws because 

of the high density of draws, resulting in a higher COPsys. Unit D had the greatest COPsys during 

the evening segment because it did not turn on its heating element and its heating capacity is low. 

The COPsys values for Draw Profile 2 are similar to the COPsys values that can be obtained from 

the COPhp curves that define the heat pump performance. There is also little difference in 

performance across the four units that performed Draw Profile 2. 

Standby Heat Loss Coefficient Comparison

The heat loss coefficient, UA, indicates how much heat is lost to the ambient air. A large value 

for UA implies a poorly insulated tank or the presence of thermal shorts. Table 5 shows the 

standby heat loss coefficient for each HPWH as calculated during the Draw Profile tests. 
52 

Table 5. Standby Heat Loss Coefficient for Each HPWH

HPWH Mfr. UA (kJ/hr/°C)

Unit A 7.11 ± 0.13

Unit B 9.60 ± 0.20

Unit C 7.39 ± 0.15

Unit D 8.74 ± 0.17

Unit E 6.23 ± 0.15

While all the UA values for the different HPWHs were found to be similar, it is also clear that 

some units are losing more heat to ambient than others. Unit B has the largest UA value, 

indicating a poorly insulating tank or many thermal shorts. On the other side of the spectrum, 

Unit E has the smallest UA, meaning that the tank is well insulated or has better management of 

thermal shorts. 

Recovery Rate Comparison 

One concern about HPWH technology is the slow rate of recovery to set point that could result in 

insufficient hot water to meet demand. As shown in Table 6, the recovery rate for a standard 

electric resistance water heater with 4.5kW of heating capacity is 22.3°C/ hr. This means that a 

standard 50 gallon water heater could recover from a series of large draws within an hour. 

Table 6. Recovery Rate Comparison vs. Electric Resistance Water Heater

HPWH Recovery Rate 

(°C/hr)

Percent 

Reduction vs. 

Electric (%)

Electric Resistance* 22.3 -

Unit A 8.5 62

Unit B 6.3 72

Unit C 13.2 41

Unit D 5.4 76

Unit E 9.6 57

*Note: From Energy Plus Simulation using 50 Gallon Electric Resistance 

Water Heater with a heating capacity of 4.5kW.

When comparing recovery rate between the standard electric water heater and the HPWHs, the 

difference is significant. Table 6 shows the recovery rates for the HPWHs when tested at 14°C 

inlet air wet-bulb temperature. The data used to calculate these values was taken from the 

operating mode Tests when water was drawn until the heat pump alone turned on and then the 

tank was allowed to recover. The data from these tests was used in place of the COPhp tests since 

the HPWH were allowed to operate as they would normally, where some controls were 

overridden during the COPhp tests. The intent was to capture heat pump-only operation, unless 

that operation rarely occurs, as with Unit C. The results show that recovery rates for HPWH vary 

between 5.4°C and 13.2°C, which correspond to a reduction in recovery rate relative to an 

electric resistance water heater of between 41% and 76%.

These reductions are significant, but are not expected to be detrimental to the technology if the 

storage tank is sized appropriately for the household. Households with large demands should use 
53 

a HPWH with a large storage tank so that hot water is less likely to run out. In addition to the 

tank size, control logic is important, as was mentioned in the section describing the operating 

mode tests for each unit. As new manufacturers enter the market and as newer versions of the 

models tested become available, it will be difficult to know how the control logic is programmed 

before purchasing a HPWH. However, as future generations of HPWHs make their way to the 

market, the control logic will inevitably improve and cater to a range of hot water demands. 

Cooling Capacity and Sensible Heat Ratio Comparison 

The operation of the heat pump removes heat from the air through a vapor compression cycle 

that is similar to a conventional air conditioner. Therefore, operating the heat pump will have a 

certain cooling effect on its surroundings. This total cooling effect can be separated into a 

sensible and latent cooling capacity for each unit, as well as a sensible heating ratio (SHR), 

which is defined as follows:

SHR = qs qt �

where qs is the sensible cooling capacity and qt is the total cooling capacity. Table 7 shows the 

sensible and total cooling capacities for each of the HPWHs tested and the corresponding SHR at 

an inlet air wet bulb temperature of 14°C (57°F) and an average storage tank temperature of 

48.9°C (120°F). Note the cooling capacity numbers and SHR will change with wet bulb 

temperature and tank temperature. 

Table 7. Cooling Capacity and SHR Comparison

HPWH 

Mfr.

Sensible Cooling 

Capacity (W)

Total 

Cooling 

Capacity (W)

SHR

Unit A 1680 1704 0.986

Unit B 672 768 0.875

Unit C 905 1350 0.670

Unit D 1015 1027 0.988

Unit E 1048 1051 0.997

Biquadratic Coefficients for HPWH Modeling

The performance mapping data acquired during testing can be used to develop and validate 

simulation models that represent HPWH technology. One way to represent the data for COPhp

and compressor heating capacity is by using biquadratic curve fits. For HPWHs, these curves are 

defined using the following equation: 

Performance Variable = C1 + C2 ∗ Twb + C3 ∗ Twb

2 + C4 ∗ Twa + C5 ∗ Twa2 + C6 ∗ Twb ∗ Twa

where Twb is the inlet air wet bulb temperature and Twa is the average water temperature in the 

storage tank. The coefficients used to define the normalized COPhp and heating capacity curves 

for the test articles can be found in Tables 8 and 9. The COPhp and heating capacity equations 

were normalized by rated COPhp and rated heating capacity, respectively. The rated values for 

each unit can be found in Table 10. The rated conditions are defined at a Twb of 14°C (57°F) and 

a Twa of 48.9°C (120°F). 
54 

Table 8. Coefficients for Normalized COPhp Curve 

HPWH 

Mfr. C1 C2 C3 C4 C5 C6

Unit A 1.229E+00 5.549E-02 1.139E-04 -1.128E-02 -3.570E-06 -7.234E-04

Unit B 1.192E+00 4.247E-02 -3.795E-04 -1.110E-02 -9.400E-07 -2.657E-04

Unit C 6.945E-02 6.601E-03 1.598E-04 8.842E-04 8.170E-06 3.255E-05

Unit D 9.814E-01 5.334E-02 -2.802E-04 -3.073E-03 -1.384E-04 -2.897E-04

Unit E 2.168E+00 8.124E-02 4.786E-04 -4.870E-02 4.284E-04 -1.499E-03

Table 9. Coefficients for Normalized Heating Capacity Curve

HPWH 

Mfr. C1 C2 C3 C4 C5 C6

Unit A 7.055E-01 3.945E-02 1.433E-04 2.768E-03 -1.069E-04 -2.494E-04

Unit B 5.050E-01 5.116E-02 -2.026E-04 5.444E-03 -1.154E-04 -2.472E-04

Unit C 6.879E-01 1.987E-02 7.659E-04 2.621E-03 5.323E-05 -5.210E-04

Unit D 5.101E-01 3.588E-02 5.563E-05 4.828E-03 -1.348E-04 7.738E-05

Unit E 9.285E-01 4.088E-02 2.737E-04 -3.625E-03 -6.521E-05 -2.986E-04

Table 10. Rated COPhp and Heating Capacity

HPWH 

Mfr.

Rated 

COPhp

Rated 

Heating 

Capacity (W)

Unit A 2.43 2350

Unit B 2.76 1380

Unit C 2.42 2670

Unit D 2.77 1820

Unit E 2.02 2040

It should be noted that these curves do not include the stand-by losses associated with the storage 

tanks. The stand-by losses were removed from the COPhp calculation for the purposes of 

modeling inputs. Also, note that it is important to use the storage tank UA values to include the

effect of stand-by losses when running an energy simulation.

A biqradratic curve fit was chosen to represent the COPhp and heating capacity data so it could 

be easily incorporated into the energy simulation software, EnergyPlus (EnergyPlus 2010). A 

HPWH model currently exists in EnergyPlus, and has been evaluated with the data from the 

results for Unit B (Hudon 2012). The results of this evaluation show that the current model 

configuration in EnergyPlus does not accurately capture the energy used by the HPWH. Efforts 

are underway to improve this model for eventual incorporation into the whole-house energy 

simulation tool, BEopt (NREL 2011). 
55 

Conclusions 

The laboratory results presented in this paper show that the HPWH is a promising emerging 

technology with the potential to significantly reduce the energy associated with generating hot 

water relative to standard electric water heaters. We conclude from these results that a HPWH

can be used to save energy in all U.S. climates when compared to an electric resistance water 

heater, and that this technology would provide the most benefit in warmer climates due to the 

improved performance and space cooling/dehumidification effects. The results of the COPhp

tests show that heat pump performance is reduced at lower air temperatures, but even at these 

conditions, power consumption is roughly one-third to one-half that of a conventional electric 

resistance water heater when using the heat pump exclusively. 

The performance results for HPWHs are promising, but another important factor is how they will

be perceived by homeowners. One issue homeowners might encounter when using this 

technology is its lower rate of recovery in heat pump-only operation. This issue can be mitigated 

by choosing a tank size that is appropriate for the hot water demand of the household. It is 

suggested that existing water heaters be replaced only by HPWHs with the same or increased 

tank capacity. In addition, the various operating modes are designed to be changed based on the 

user’s current need. Homeowners should become familiar with the user manual for the unit they 

purchase so the correct operating mode is selected. 

Homeowners and installers must also be aware of the physical challenges associated with 

installing these units as well as their effect on the surrounding environment. HPWHs are larger 

than conventional water heaters and require a sizeable volume of air from which to draw heat. 

This imposes limits on the installation location and should be considered before purchasing a 

unit. If the physical requirements can be met, the surrounding environment must also be 

considered. In regions with a long heating season, it is not recommended that HPWHs be 

installed in conditioned space, particularly if space heating is provided via electric resistance.

The garage could be an ideal location for this technology if freezing temperatures are not 

common. In addition, installation in an unconditioned basement can be considered as long as 

cooling the basement air is not a concern. Operating the heat pump can dehumidify the air 

around it, which could be an advantage in a basement environment. One other installation 

consideration is the presence of a drain. Operating the heat pump will result in condensation 

building up on the evaporator coils that needs to be directed to a nearby drain. 

Some of the issues that were found with the HPWH technology can be improved upon as future 

generations are released. Three areas where improvements could be made include control logic, 

user interfaces, and ease of serviceably. It was found that the COPsys of the HPWHs operating in 

their Hybrid modes varied across the models tested, mostly as a result of the differences in 

control logic. For example, some control schemes appeared to sacrifice efficiency in favor of 

shorter recovery times, while other strategies maintained efficiency by minimizing the use of the 

electric elements, which limited the volume of hot water available during high demand times. In 

addition to control logic, some of the units have user interfaces that are not intuitive or require 

toggling through multiple menus to make changes. This issue could be frustrating to a user, and 

result in a homeowner not changing modes at all. Also, if there is an issue with the operation of 

the heat pump, it is possible for the homeowner not to realize it because hot water will still be 
56 

available as long as the back-up elements are functional. An effective means of detecting and 

informing the homeowner of faults or necessary service by an HVAC technician should be 

included on every unit. 

Selecting the correct HPWH for a specific application can be challenging since advantages and 

disadvantages exist for each of the units that were tested. The results of this research are intended 

to provide guidance to homeowners and builders who are considering purchasing and installing a 

HPWH in a residential home. The following paragraph summarizes the findings of this research 

for the specific units that were tested.

In general, we found that Unit A performed well and was able to provide ample hot water 

because of its large tank volume and responsive control logic. The drawbacks of this unit include 

its high airflow rate and size, which may result in installation difficulty. Installation location 

must be considered carefully for this unit. Unit E performed well at low air and water 

temperatures and its exhaust airstream can be easily ducted. This makes it a good option for cold 

climates. The drawbacks to this unit are the non-intuitive user interface, the inaccessible air 

filter, and lack of heat pump access for servicing. Unit B also performed well, but was not as 

consistent at delivering hot water to the end user because of its smaller tank volume. For low 

demand situations or when tank size is an installation concern, Unit B would be a good option. 

Unit D was the most efficient of the HPWHs tested, but it also was the slowest unit to recover, 

the electric resistance element was not helpful in high-demand situations, and the set point is 

difficult to change. In addition, it only has one operating mode. Unit C performed well, but only 

within a limited range of operating temperatures. Due to this limitation, this unit is expected to 

be less efficient when used in cold or hot climates unless installed in conditioned space. Due to 

its small storage tank and water circulation pump, this unit had difficulty maintaining hot water 

in high demand situations. 
57 

References

Ashdown, B.G. (2004). Heat Pump Water Heater Technology: Experiences of Residential 

Consumers and Utilities. Oak Ridge National Laboratory, Oak Ridge, TN. Accessed 

at: http://web.ornl.gov/~webworks/cppr/y2005/rpt/120517.pdf

Calm, J.M. (1984). Heat Pump Water Heaters. Electric Power Research Institute (EPRI), Palo 

Alto, CA.

Consumer Energy Center. (2011). Water Heaters. Retrieved May 16, 2011, 

from http://www.consumerenergycenter.org/home/appliances/waterheaters.html

Department of Energy. (1998). 10 CFR Part 430 Energy Conservation Program for Consumer 

Products: Test Procedure for Water Heaters, Federal Register Vol. 63, No. 90. 

EnergyPlus. (2010). Version 6.0 Documentation. 

Hendron, B.; Burch, J.; and Barker, G. (2010). Tool for Generating Realistic Residential Hot 

Water Event Schedules. NREL/CP-550-47685. Golden, CO: National Renewable Energy 

Laboratory. Accessed at: http://www.nrel.gov/docs/fy10osti/47685.pdf

Hudon, K.; Sparn, B.; Christensen, D.; and Maguire, J. (2012). Heat Pump Water Heater 

Technology Assessment Based on Laboratory Research and Energy Simulation Model. 36 pp.; 

NREL Report No. CP-5500-51433. Prepared for ASHRAE Winter Conference 2012, Jan. 21-25, 

2012. Chicago, IL. Accessed at: http://www.nrel.gov/docs/fy12osti/51433.pdf

Hashimoto, K. (2006(). Technology and Market Development of CO2 Heat Pump Water Heaters 

(ECO CUTE) in Japan. Central Research Institute of Electric Power Industry. Nagasaka, Japan.

Levins, W.P. (1982). Estimated Seasonal Performance of a Heat Pump Water Heater Including 

Effects of Climate and In-House Location. Oak Ridge National Laboratory, Oak Ridge, TN. 

Accessed 

at: http://btric.ornl.gov/eere_research_reports/appliances/water_heaters/heat_pump_water_heater

s/ornl_con_81/ornl_con_81.html

Morrison, G.L. (2003). Seasonal Performance Rating of Heat Pump Water Heaters. School of 

Mechanical and Manufacturing Engineering, University of New South Wales, Sydney, Australia. 

Accessed 

at: http://www.ecaa.ntu.edu.tw/weifang/hp/heatpumpEng/Seasonal%20performance%20rating%

20of%20heat%20pump%20water%20heaters.pdf

NREL. (2011). Building Energy Optimization (BEopt) Software. Golden, Colorado. National 

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58 

Appendix A – Draw Profiles

Table 11. Draw Profile 1 Details. Morning Segment Includes the Four Showers and the Evening 

Segment Contains Remaining Draws

Draw Profile 1

Start 

time

End 

Time

Flowrate 

(lit/min) Liters/draw Notes

6:10 6:19 7.6 68.1 Shower 1

6:30 6:35 7.6 37.9 Shower 2

6:50 7:00 7.6 75.7 Shower 3

7:10 7:18 7.6 60.6 Shower 4

17:03 17:04 3.8 3.79

Food Preparation

17:04 17:05 1.9 3.79

17:05 17:06 3.8 1.89

17:09 17:10 3.8 3.79

17:10 17:11 5.7 3.79

17:12 17:14 3.8 5.68

18:10 18:11 3.8 7.57

Hand Washing 18:11 18:12 1.9 3.79

18:12 18:13 5.7 1.89

18:13 18:14 3.8 5.68

18:45 18:48 7.6 3.79

18:55 18:57 7.6 22.71 Dishwashing

19:10 19:12 7.6 15.14

19:50 19:54 11.4 15.14 Bath

20:50 20:51 1.9 45.42

Face Washing 20:51 20:52 3.8 1.89

20:52 20:53 1.9 3.79

20:54 20:55 3.8 1.89

Figure 40. Graphical representation of Draw Profile 1
59 

Table 12. Draw Profile 2 Details

Draw Profile 2

Start 

Time End Time Flowrate (lit/min) Liters/draw

6:00:00 6:00:30 3.8 1.893

6:10:00 6:10:30 3.8 1.893

6:20:00 6:21:00 3.8 3.785

6:35:00 6:35:30 3.8 1.893

6:50:00 6:50:30 3.8 1.893

7:05:00 7:06:00 3.8 3.785

7:10:00 7:11:00 3.8 3.785

7:55:00 7:55:30 3.8 1.893

8:05:00 8:05:30 3.8 1.893

8:20:00 8:20:30 3.8 1.893

8:35:00 8:35:30 3.8 1.893

8:45:00 8:45:30 3.8 1.893

8:50:00 8:50:30 3.8 1.893

9:05:00 9:05:30 3.8 1.893

9:10:00 9:10:30 3.8 1.893

9:15:00 9:15:30 3.8 1.893

9:25:00 9:26:00 3.8 3.785

9:30:00 9:31:00 3.8 3.785

9:35:00 9:36:00 3.8 3.785

9:40:00 9:41:00 3.8 3.785

10:50:00 10:51:00 3.8 3.785

10:55:00 10:56:00 3.8 3.785

11:00:00 11:00:30 3.8 1.893

11:05:00 11:05:30 3.8 1.893

11:30:00 11:31:00 3.8 3.785

11:35:00 11:36:00 3.8 3.785

11:40:00 11:41:00 3.8 3.785

11:45:00 11:45:30 3.8 1.893
60 

Figure 41. Graphical representation of Draw Profile 2
61 

Appendix B – Laboratory Schematics

Figure 42. Water-side schematic Ice Machine

Neslab 750HX Chiller

Chromalox 

CMX-250-9

Heater

Pump

Mains Water

Water Dump

Water Dump

Glycol to Water 

Cooling HX

Ball

Valve

Solenoid

Valve

Solenoid

Valve

Supply Water Tank

HPWH 1

6 TC

HPWH 2

6 TC

On/Off

Solenoid

Valve

Turbine 

Flowmeter

Coriolis meter 

for Condensate

T

P,T

T

Coriolis meter 

for Condensate

Back 

Pressure 

Valve

T

T

P

T

P

T

P

Turbine 

Flowmeter

Flow Control

Valve

T P

Turbine 

Flowmeter

Ball

Valve

Ball

Valve

Turbine 

Flowmeter

On/Off

Solenoid

Valve

Flow Control

Valve
62 

Figure 43. Air-side schematic

Outside Fan

Heaters

Evaporative pads

Heaters

18 in Ducts

Heater

HPWH1

 test chamber

HPWH2

test chamber

LFE

LFE

Excess Air

Excess Air

Neslab 750 Chiller

Outside Fan Outside Fan

To Outside

To Outside

INLET 1

OUTLET 1 OUTLET 2

T

Flow Nozzles

VFD

VFD

Liquid-to-air 

cooling HX

P, T, H

P, T, H

Flow Nozzles Flow Nozzles

P, T, H
63 

Appendix C – Heating Capacity Curves

Figure 44. Heating capacity as a function of average tank temperature for Unit A

Figure 45. Heating capacity as a function of average tank temperature for Unit B
64 

Figure 46. Heating capacity as a function of average tank temperature for Unit C

Figure 47. Heating capacity as a function of average tank temperature for Unit D
65 

Figure 48. Heating capacity as a function of average tank temperature for Unit E